Vioxx was heavily marketed as the safest and most effective drug for arthritis and related conditions when it was launched in the UK six years ago. Arthritis Care, one of the biggest charities in the field, received ?26,000 from Merck in 2004 alone.

Other recipients included the National Rheumatoid Arthritis Society, the National Osteoporosis Society and the British Society for Rheumatology.

Experts said that the relationship between Merck and arthritis groups was merely a small part of a bigger problem, with millions of pounds being spent by drugs companies in return for their corporate logo and name being featured on websites and literature created by respected and influential charities.

Sapna Malik, a solicitor representing more than 50 patients who are suing Merck over Vioxx, said: "I am sure the charities would say that they act independently of the companies that give them money, but it is an unhealthy relationship.

"How can a charity that has taken money from Merck and features its logo, then offer impartial advice if something goes wrong? Drugs companies want to get something for their money, and the emerging situation surrounding Merck has brought the way in which this can prove to be a conflict of interest to the fore."

Vioxx was launched in the UK in 1999 and it was aggressively marketed to GPs, hospital consultants and patient groups as a new type of painkiller that was more effective and safer than old-style anti-inflammatory drugs. It promised to be particularly useful for arthritis sufferers.

But it was withdrawn in September last year after it emerged that it was linked to a doubling of heart attacks and strokes in patients. Merck is now facing legal action from more than 4,000 patients around the world, including 300 in Britain, who claim that they or their relatives suffered or died as a result of Vioxx.

It is estimated to have contributed to the deaths of around 60,000 people.

Merck has also been hit by allegations that it knew as early as 1998 that the drug carried increased risks of cardiovascular problems, but withheld the information from both the US Food and Drugs Administration and the UK's Medicines and Healthcare Products Regulatory Agency (MHRA) when applying for licences.

The Independent has now learnt that many of the biggest arthritis charities have been long-standing recipients of sponsorship and donations from Merck's UK subsidiary, Merck Sharp and Dohme (MSD). The website for Arthritis Care says its creation was "made possible with support from Merck Sharp and Dohme".

Arthritis Care was one of several organisations that submitted evidence to the NHS drugs assessment body, the National Institute for Clinical Excellence (Nice), in 2000 when it was investigating how widely Vioxx should be prescribed in Britain. Since the scandal surrounding Vioxx emerged, Arthritis Care has said it cannot comment until the MHRA has concluded its investigations. The charity accepted ?10,000 from MSD in 2003 and ?26,000 last year, a spokesman said.

He added: "Arthritis Care does not advise or recommend specific medications to people with arthritis. As a patient representative body we provide factual information on living with arthritis. Arthritis Care does not moderate its messages according to the wishes of external agencies."

A report by the National Rheumatoid Arthritis Society in 2003,The Painful Truth, called for faster access to new medication and was sponsored by MSD, which it names on the title page. A spokeswoman said the charity had "clear rules" about how it accepted money from the industry and that it would refuse to work with any company that acted deceitfully.

The British Rheumatology Society, which represents clinicians and scientists, also has MSD as a commercial sponsor. David Isenberg, the BRS's president, said it did not accept money for endorsing drugs or companies.

Other organisations were unwilling to publicly criticise the arthritis charities. One insider said: "It can be difficult as you desperately need the money and these drugs companies come offering it on a plate, but I think it's more trouble than it's worth getting into bed with the pharma industry ... And it's not just arthritis charities - it's going on everywhere in the voluntary sector, although it's not shouted about. That tells you how murky it can be."

MSD was asked to comment on its donations and links to charities, but the company did not respond.

'Company should be in dock'

Donald Clark, 69, husband of Vioxx taker

Irene Clark was a fit and healthy grandmother who spent every lunchtime supervising children in the playground of a nearby primary school.

Yet within two months of being prescribed Vioxx, the 62-year-old suffered a massive heart attack and died.

It was only when the drug was withdrawn in September last year that her husband, Donald, made the link with his wife's death. Mr Clark, 69, from Gillingham, Kent, has now joined the legal action against Merck, with hundreds of other British people.

He said: "I blame myself because I suggested she went to the doctor and asked for different medication because the old drugs weren't doing her any good. It's a nightmare. The drugs company should be up in the dock for murder."

Mrs Clark had suffered from rheumatoid arthritis in her knees and shoulders for some years. Her GP prescribed her Vioxx, in a liquid rather than tablet form. It worked well, and Mrs Clark obtained a repeat prescription.

On 1 August 2003, Mr Clarkfound her slumped at the top of the stairs in their home. "The doctor said she had had a massive heart attack. Irene had never had any heart trouble," he said.

A year after Mrs Clark's death, Merck voluntarily withdrew Vioxx from the market. It was only then that Mr Clark made the link.