Drug companies are spending millions of pounds every year on all-expenses-paid trips to conferences around the world for doctors and other hospital staff, in what critics say is a massive marketing exercise dressed up as medical education.

The Guardian can reveal the scale of pharmaceutical company sponsorship following an examination of the registers of gifts and donations to doctors that all hospitals are required to keep. They show considerable largesse - from drug companies regularly picking up hefty bills for travel to international conferences in Europe, Asia and America, to specialist nurses' salaries, and weekly sandwich lunches for hospital staff training sessions.

All-expenses-paid trips to conferences in the US, Vietnam or Hungary are a regular feature of the registers, costing the companies up to £5,000 per doctor. Many of the declarations by doctors do not put a price on the trip. The total amounts received by staff at individual hospital trusts with complete registers are substantial - Sheffield's staff received funding of more than £105,000 from pharmaceutical and medical devices companies in the 12 months to last June.

Examples of the firms' hospitality include:

· Astra Zeneca paid £2,500 for a doctor at the Royal Bournemouth trust and £1,500 for a doctor at Sheffield teaching hospital to attend a cancer conference in Texas

· Sanofi-Aventis, the world's fourth biggest pharmaceutical company, paid for doctors at the Countess of Chester trust to go to conferences in Cape Town, New Orleans and Barcelona. At Gateshead trust, their reps gave a breakfast for 30 staff "to discuss drugs for the treatment of breast cancer". The trust's register records that "the donor was seeking to secure business".

· Roche spent £2,000 for an oncology consultant at Addenbrooke's hospital in Cambridge to go to a conference in May last year.

· GSK, the biggest British pharmaceutical company, paid £1,200 for a consultant at Sheffield teaching hospital to attend the 11th international congress of Parkinson's disease and movement disorders in Turkey last June.

· Companies have also been taking hospital staff to top football and rugby matches. Carillion, a public sector construction firm, spent £180 taking a senior manager at Milton Keynes trust to lunch and then a rugby match at Twickenham last August.

Most doctors deny that their reliance on drug company cash makes them biased. The pharmaceutical companies argue that they are helping doctors acquire further medical education by funding their trips to conferences in foreign cities, but they refuse to reveal how much they pay out.

However, the health select committee warned in a report in 2005 that the industry's sponsorship of doctors and other medical staff had drug promotion as its motive and could lead to unsafe prescribing of drugs such as Vioxx, the arthritis drug which was found to cause heart attacks.

Joe Collier, the recently retired professor of medicines policy at St George's hospital, London, a former member of the Medicines Commission and an adviser to the select committee, said: "Through its orchestrated campaigns affecting all those involved in the use of medicines, the pharmaceutical industry enormously influences what patients are prescribed. On the whole these influences are detrimental to best practice."

Payments to doctors are far from transparent. The Department of Health requires NHS trusts to compile registers of their medical staff's and directors' possible conflicts of interest and to make them available to the public. Only a minority do so. The Guardian requested the registers for 90 hospital trusts under freedom of information legislation. Only around a quarter returned data that included the names of the doctors and the sponsoring companies and the amounts of money received. Some refused to give any information at all.

Collier said this was unacceptable. "Declarations of interest are a key way to help break the pharmaceutical industry's stranglehold. It is not a trivial issue. Public declarations by doctors are essential if prescribing is to be sensible and appropriate and according to patients' needs."

Consumers International (CI) said the lack of transparency was unacceptable. "When a medical professional speaks on a health issue, we assume that they are putting patients' interests first. If that person has a conflict of interest because they or their organisation are receiving funding from a drug company the least we should demand is the right to know about it," said Justin Macmullan, head of campaigns. "Pharmaceutical companies will tell you that what they are funding is medical education. But our concern is that this is really highly effective, well-targeted marketing. This throws any notion of impartiality out of the window and jeopardises a doctor's ability to make an informed, balanced decision about the most appropriate treatments."

CI wants drug companies to declare how much they give to doctors. "Countries such as the US and Australia have woken up to this issue and are now requiring pharmaceutical companies to disclose their funding of medical organisations and medical education. European regulators have been sleeping on the job," he said.

Labour MP Paul Flynn described as "codswallop" the companies' claim that their only intention was to help educate doctors. "It's not true. It's part of a huge marketing budget. It's all about maximising their profits, not helping people in life-threatening situations," he said. "The influence of these companies is enormous."

Doctors who receive funding believe they are not influenced by it. Robert Storey, a consultant cardiologist at Sheffield involved in drug trials, took four trips to conferences in the year to June 2007 courtesy of Astra Zeneca at a total cost of £12,000. However, he regards these as business trips because he is asked to disseminate research findings and are funded from the R&D budget. More junior doctors have their funding arranged through the drug rep and must fly economy class under Association of the British Pharmaceutical Industry (ABPI) rules.

"If it is done through the local rep, who may expect some sort of favour in exchange for that sponsorship, there is more stringent regulation," said Dr Storey. "[Those doctors] are seeing reps on a regular basis and although it is explicitly stated in the ABPI rules that there shouldn't be any conflict or conditions [on the funding], it probably does influence doctors' behaviour because they are unsure whether they will get further sponsorship for going to further meetings, so it is useful to them to engender good relationships with different reps.

"One has to be careful how one judges oneself, but when you get to my level where you are getting a lot of interest from different companies, you can pick and choose to a certain extent. If you feel uncomfortable about any particular request or association, you can very easily walk away. If I'm asked to put certain points across in a talk which I think are biased, I won't do it in the best interests of patient care."

Storey, who makes a fuller declaration than most doctors, would prefer to see a different system. "I certainly think it would be preferable if sponsorship or money for travelling to meetings was independent from the pharmaceutical companies but there is no pot of money for providing that," he said.

Dr Willy Notcutt, an expert in pain relief at James Paget hospital in Great Yarmouth, has recently returned from a big conference in Glasgow. Two companies, Eli Lilly and Boehringer, paid £800 for his travel, accommodation and registration fee. He says he has been prescribing a drug sold jointly by the two companies but was not "brainwashed" into it by their hospitality. He made his own independent evaluation of the merits of the drug. "I don't give a toss what the drug company rep says. I prescribe drugs which give benefit to my patients," he said.

The ABPI said doctors would not be able to attend conferences where they hear from experts in their field without sponsorship, but it was important this was transparent.