Cancer patients who cannot get the modern drugs they need on the NHS are ordering them directly from international 'internet pharmacies', often without their doctors' knowledge.

Patients are beginning to self-prescribe cancer treatments by ordering them online, after learning about the newer therapies, such as Avastin for bowel cancer and Tarceva for lung cancer. Many treatments have not yet been licensed by the government's watchdog, the National Institute for Health and Clinical Excellence (NICE), but are widely promoted on the internet.

One of Britain's leading cancer specialists, Karol Sikora, told a meeting of politicians and doctors last week that he had a number of private patients who had begun to order drugs from one of the largest internet pharmacies in Canada, CanadaDrugs.com. 'These patients are well informed, and they shop around for the cheapest prices,' he said. 'I had one patient, a very well educated young woman who wanted Tarceva for lung cancer. She couldn't get the drug on the NHS. The price from a Harley Street clinic worked out at around £75 a tablet - but ordering from Canada would bring it down to £35 a tablet.'

He told the conference that a younger generation of patients would demand the best treatments, even if the NHS decided they were not affordable. 'We're in a world where an easyJet flight can be booked within minutes. The idea that you can't get a cancer drug that will extend your life because a government body has decreed you can't have it just won't work.'

In the past, the worries have been about patients ordering so-called 'lifestyle' drugs online to reduce weight, cope with depression or boost sex life. Viagra or Cialis for impotence, Prozac for depression and Reductil for weight loss have been widely ordered online over the past five years. But there are concerns over safety when consumers move into ordering more mainstream medicines.

One of the biggest exporters is Canada Drugs, a company in Winnipeg that sells both prescription and over-the-counter medicines online. It offers Tarceva for lung cancer, but also asks patients to provide a note from their doctor (or details of their doctor's surgery) as part of their application for buying the drug.

The company exports more than 3,000 prescriptions a day around the world, mostly to America, where medicines are far more expensive. An attempt by President Bush to crack down on the use of such companies failed after pressure from senators and the public.

The Food and Drug Administration still intercepts thousands of prescriptions ordered from Canada that are sent in the post. It was announced yesterday that US policy will be changed, so that customs agents will focus only on intercepting large imports, rather than the smaller packages that are often for individual use by the elderly who cannot afford the medicines from any other source.

A spokesman for CanadaDrugs.com told The Observer: 'Britain has so far not been a big market for us, but we are getting more customers now. At the moment it's only a few UK clients each month, but we think it is working by word of mouth. People know that our drugs are from reputable suppliers and that we are efficient.'

Many cancer specialists question whether all the internet sites actually demand a bona fide prescription from an oncologist. They urge patients not to go to internet suppliers for their drugs. Professor John Toy, medical director of Cancer Research UK, said: 'It is very worrying that patients may be purchasing medicines online to treat themselves. Anyone thinking of procuring their own drugs should speak to their cancer doctor, as it could interfere with their treatment programme and could do them more harm than good.

'Cancer is a complex illness and requires specialist knowledge and expertise. The drugs involved are extremely sophisticated and potentially dangerous, and therefore should always be given under close medical supervision.'

The Medicines and Healthcare Regulatory Authority is also aware of the trend, but is powerless to intervene. A spokesman said: 'We do have worries. Patients should ask themselves where the medicines come from, and we know that, while some websites are lawful, other are operated by criminals.'

But in the UK there is little that can be done to stop the move. The Royal Pharmaceutical Society of Great Britain, which represents UK pharmacies, has been working on ways of advising patients how to tell if an online pharmacy is safe and properly regulated - but it only applies to British-registered pharmacies. The society's spokeswoman said: 'Sadly, we have no influence or say over what happens abroad. We can only warn people about the dangers. Our message is simple - don't do it.'

However, the number of cancer patients looking to find different ways of ordering drugs is likely to grow, as new and expensive therapies appear for which there is no funding. Last month, three bone and blood cancer victims began to campaign over Velcade, a drug not licensed on the NHS because NICE has stated it is not cost-effective at £18,000 a course, as it can prolong the life of a multiple-myeloma patient only by a few months, not years. Jacky Pickles, 44, Janice Wrigglesworth, 59, and Marie Morton, 57, all have the disease and are demanding the drug.

Other new cancer drugs are coming on stream next year, such as Tykerb, a new tablet for breast cancer, which has shown to work in women whose cancer is resistant to the drug Herceptin. It is likely to be just as expensive as Herceptin, at about £20,000 a course. In the case of Tarceva, a drug for lung cancer patients, its full assessment by NICE is not even scheduled to happen for another year. In some patients Tarceva can prolong life by up to 18 months.