Critical illness cover used to be the bad boy of the protection insurance world. This is a type of insurance that should pay out a lump sum if you are diagnosed with a life-threatening illness such as cancer or heart disease. The money is tax free and can be used in any way you like – to pay a lump sum off your mortgage, enable you to stop working, or buy private medical care. You decide how much you think you'll need when you take it out.

In the past, these protection policies were slated for having too much wriggle-room, allowing insurers off the hook when it came to paying claims – and insurers were notoriously secretive about how many claims they let through. Not surprisingly, insurers found critical illness cover increasingly hard to sell, and so have been forced to clean up their act. First, they started to declare how many claims were successful – they say that, on average, more than nine in 10 are now met. Then they increased the number of illnesses and conditions covered. Some covered fewer than 10, but the list now typically covers 40 to 50.

Tom Baigrie, at independent protection insurance broker LifeSearch, says: "Historically, policies only covered advanced cancers, which meant many early-stage cancers were not covered, but in recent years this has changed."

Cancer accounts for almost two-thirds of claims, followed by heart attack, stroke and multiple sclerosis. But it's important to remember that critical illness insurance only covers the conditions laid out in the policy and no others, and your illness must meet the tight definitions described.

Do you need it?

It is worth first checking what benefits you have through your work. Most employers offer life insurance (which only pays out on death) and few include critical illness cover. Experts say you are six times more likely to claim under critical illness than life insurance.

If the reason you are thinking of taking out critical illness cover is to pay for private treatment, you should, instead, consider opting for private medical insurance. However, because a critical illness payout has no restrictions on how you can use it, it could be useful to help pay your mortgage, or allow you to go part-time, and generally ease your household finances.

Critical illness insurance is complicated and you should talk to a specialist adviser before choosing a policy.

How much does it cost?

Critical illness is usually added to life insurance, and the cost will depend on your age and medical history. The cheapest policies have the fewest conditions covered and the tightest definitions of when it will pay a claim. You can keep down the cost at the start by going for a reviewable plan. However, the premiums will then be raised every few years, and it could work out more expensive in the long run than a guaranteed plan, which keeps the premiums level throughout.

The cost of £100,000 life and critical illness cover over 20 years for a non-smoker in good health with guaranteed rates would be £21.29 a month with Aegon for a 30-year-old; £42.91 for a 40-year-old with Legal & General; and £104.41 for a 50-year-old through Ageas, according to LifeSearch.

Since critical illness cover is the most expensive element, you can keep the price down by opting for different levels. So someone with £100,000 life insurance and £50,000 of critical illness cover over 20 years, would pay £13.55 a month with Zurich at age 30; £26.75 with Aegon at 40; and £61.41 with L&G at 50, according to independent adviser Alan Lakey at CIExpert.

Why are claims rejected?

Two of the most common reasons claims are declined are that they don't meet the definitions covered by the policy and/or that the policyholder allegedly didn't tell the insurer about a previous medical problem.

The Association of British Insurers (ABI) has produced guidelines for providers aimed at making sure the definitions are clear.

Ben Heffer, at independent financial researchers Defaqto, says: "During the year we have seen enhancements to the cancer definition, blindness, third degree burns, benign brain tumour and major organ transplant."

Since insurers don't undertake thorough medical checks when they sign you up, it is easy to fall foul of the system accidentally and miss something off your medical history. Even a minor incident in the past could mean insurers refuse to pay.

But claims following innocent or inadvertent mistakes will be met, at least, in part. For instance, last year 1.7% of Aviva's critical illness claims were declined due to non- disclosure and 92.5% were successful.

Earlier this year, Aviva asked customers about their understanding of critical illness cover. It found 45% thought it would cover any critical illness, not just those listed under the plan.

Partial payments

The latest move is to include "severity-based" cover. This is where plans pay out a proportion of the sum assured, depending on the severity of the diagnosis, and continue to provide cover for the more serious conditions.

Heffer says: "There is a strong trend towards adding severity-based payments. In May 2012, some 10 different severity-based covers were offered, and now there are almost 30 named conditions.

Principally this relates to breast cancer and low-grade prostate cancer which may or may not require surgery. Also, other less severe cancers are covered, as well as a range of less serious conditions like a minor stroke or heart attack or angioplasty."

As advances in medical treatment improve recovery rates, this is a step in the right direction. But you need to check what a partial payment will do to your cover. Most policies will pay out between £12,500 and £25,000 – or around 20% to 25% of the sum insured, says Heffer. So if you had £200,000 cover, you would get between £40,000 and £50,000. In most cases this is an extra payment on top of the £200,000 you would receive if you later developed another serious illness.

With other policies, the partial payment is knocked off the original sum, leaving you with £160,000 to £150,000 of cover, in the example above. This is how PruProtect's policy works, though it is possible to protect the full sum by paying a higher premium of about 10% more. Also, as Heffer points out, the Pru "is far more likely to pay out for a range a critical illnesses of varying severity" as it has the biggest list of 166 conditions.