When prospective employers asked questions about her health, Ruth Rosselson felt she had to sell herself harder to compensate. "I felt I had to defend myself and my condition when I was supposed to be applying on merit," says the 40-year-old, who has palindromic rheumatism, a variable autoimmune arthritic condition. "It doesn't mean I don't work hard."

For Rosselson, an ethical consultant and writer from Manchester who is now self-employed, pre-employment medical questions proved intrusive and discouraging. "I'd start to fill them out, then end up just giving up," she says. "If an application had an extensive medical questionnaire, it put me off. If I didn't have a medical condition, it would have been much easier.

"Those sorts of questionnaires don't seem to account for the fact that a condition can fluctuate," she adds. "Either you have a disability or you don't. There are days when I can't walk, but they're very rare. My arthritis can affect my hands and how I type, but it's rare that it's that bad – maybe only once or twice a year. A lot of the time, I am just like anybody else."

Employers are no longer free to ask such questions as they please. Under the Equality Act 2010, which came into force on 1 October, any medical questions asked before a job offer is made must relate specifically to the tasks involved in carrying out the role. This is potentially good news for people who, like Rosselson, feel that disclosing information about a condition or disability may weaken their chances of getting a job.

For Peter Clark^*, an arts worker from north-east England, it's game over if an application asks how many sick days he's taken. "I once put how many I'd had and didn't get an interview for a job where I met all the criteria," he says. Clark has multiple disabilities including ataxic cerebral palsy and a digestive disorder, which once caused 17 absences in a 12-month period. "Now I know the number is too high, so there's no point even trying."

In 1998, disabled people were twice as likely as the able-bodied to be out of work. In 2010, that's still the case. Only one in 10 tribunal claims made under the Disability Discrimination Act – which the Equality Act mostly replaces – concerns recruitment, but discrimination there isn't less common – just harder to prove.

As well as limiting the questions employers can ask, the new legislation shifts the burden of proof on to the employer. It also covers a number of other areas including gender pay discrepancies, pay secrecy and protection for breastfeeding mothers.

"Discrimination legislation has developed piecemeal since the mid-70s," says Paul Gaff, a partner at law firm Thomas Eggar. "This is putting it all in one place and making sure the concepts are uniform across different types of legislation. And it's making sure employers aren't being lazy in their application processes."

The UK has lagged behind countries including France, Spain, Italy, Belgium, Portugal, the Netherlands and the US, which have established legal restrictions on pre-employment medical checks. This is the first time UK employers have been subject to such limitations, despite recommendations by the Disability Rights Taskforce in 1999 and the Disability Rights Commission in 2003.

"The law has pushed me to make career decisions I might not have had to make otherwise," says Louise Bolotin, a 48-year-old journalist and author from Manchester who has epilepsy. "People have all kinds of preconceptions about epilepsy. There is still a huge stigma attached to it."

Since she wrote a book about living with epilepsy, her CV in effect invites employers to mention the condition during interviews – giving her the opportunity to talk about it if she wishes. But, she says, disclosing a medical condition during the recruitment process should only ever be a matter of choice.

Previously, jobseekers weren't obliged to disclose medical information. Not in theory. But employers could force them to do so by making such questions a compulsory part of the recruitment process.

"I would prefer to discuss it in person after an offer of employment," says Amy Beaumont, a 22-year-old illustrator from London who has idiopathic hypersomnia, a sleep disorder. "With a box in the initial application, it shuts the door to that possibility and potentially reflects badly on me.

"I find it hard to believe that any employer would treat my application equally if I told them about my condition," she says. "Equally, I feel it must be better to be honest and upfront about it. I've found it impossible to weigh up whether I should include it in my applications or not."

Once medical questions are asked, untruthful answers may lead to withdrawal of a job offer or even legal action, as in the case of Cheltenham borough council, which in 2009 lost a £1m legal claim against former managing director Christine Laird. While the council said Laird should have disclosed previous depression in a pre-employment questionnaire, the high court disagreed. The council may have lost its case, but it's still a pertinent example of why health questionnaires may discourage disabled applicants.

All of the disabled people who spoke to the Guardian about the new legislation were keen to emphasise that having a disability doesn't mean an applicant is going to be less dedicated, productive or punctual.

"I have interviewed a great many people over the years, sometimes resulting in employing the wrong person and then needing to part company within the first six months," says Mick Say, who runs business consulting company Full Column. "That was never due to medical or health reasons. Medical questionnaires put people in a difficult position – if they are returned without completion, recruiters may think the applicant had something to hide."

Comprehensive questionnaires can also prove more or less useless to employers, says Joanne Heslop, occupational health manager at Leeds Metropolitan University, who says she also has reservations about employers tasking HR staff, not occupational health specialists, with administering medical questionnaires.

Leeds Met does not ask health questions until job offers have been made, and it has reduced the amount of information it requests. "Traditional medical questionnaires were a waste of time and paper storage," Heslop says. "Most people answer no to almost every question. We're changing it to just four questions, asking about the adjustments people need to do the job."

However, some employers are concerned the new legislation does not leave enough room for positive discussion of disability. Sonia Sharples, HR director of Poundland, says where there are barriers preventing people from entering the workplace, "you need to understand what those barriers are and help people overcome them".

Poundland does not believe in using pre-employment screening, although the company frequently recruits through Remploy, which helps disabled people to find jobs. "Our general labour turnover is 40%. For those who come through Remploy, it's 5%," she says. "But there will always be employers who take a negative view, whatever legislation you put in place."

"People need clear rights enshrined in law, but to support those rights, there needs to be very good quality assessment of what people can do and what support they need," says Sally Burton, chief executive of the charity Shaw Trust, whose remit includes helping disabled people into work. "There is a lot of prejudice and ignorance. For example, we've done employer research around mental health and found that many employers still don't want to put people who've had mental health conditions in a customer-facing role."

But keeping someone's medical history a secret isn't necessarily the answer, she says. "We get a significant number of candidates into jobs by working closely with employers."

The new legislation may spook some employers, she adds. "What's needed is really good, sophisticated training at the HR and communications level. Employers have got to be able to ask people what help they need and how best to achieve that."

Flash Bristow is a 34-year-old website accessibility specialist from Leytonstone, London. She says: "I would like the option to be there for someone to ask: is there anything you'd like to tell us?

"It's all very well pussyfooting around it. Changing what can or cannot be asked is not going to change people's attitudes. It's just going to make it more difficult to be open and honest as they are having to tiptoe round the issue while I'm sitting there with a stick – and they can't ask me about it."

Others are not convinced the legislation will make much difference. "It's not legal to ask women if they're planning to have children in the next few years, but it still happens," Clark says. "If employers are not allowed to ask about periods of ill health, they can just use the probation period to get rid of people."

^*Name has been changed

What the Equality Act says about health-related questions

• Are pre-employment medical questions banned? No, says lawyer Paul Gaff. "But questions must relate to the parts of the job that are intrinsic. Employers still have a duty to make reasonable adjustments, so they can ask if you require any to attend the interview."

• How clear are the rules? Not very. "The government is not giving definitive guidance on what questions are appropriate. Some employers will be test cases, which is really not helpful. But those who are sensible will say, you know what, I don't want a disability discrimination claim."

• What about asking how many sick days you've taken? "That falls into a grey area, but an employer would have a higher risk profile if they asked."

• Can employers ask questions about physically obvious disabilities during interviews? "The only thing they should do is ask a generic question, such as: 'Would you foresee the need to make any reasonable adjustments to facilitate your performance of the role?' And leave it there."

• What if you disclose a disability after accepting the job and the employer wants to reject you? "If you reject someone at the application stage, that's probably one of a number of rejections." Once a job offer has been made, "you're going to need to work pretty hard to show you're justified".