The British food industry has embarked on a huge lobbying campaign in Whitehall to see off growing pressure for regulation to tackle obesity and diet-related diseases.

As an influential committee of MPs prepares to publish a hard-hitting report tomorrow demanding urgent government intervention to tackle the obesity epidemic, the Guardian has obtained documents revealing the scale of the food industry's efforts to convince ministers that only voluntary, industry-controlled measures are taken on public health.

Companies are reticent about their lobbying, but the documents released to this newspaper under the open government code show how manufacturers and advertisers have been using their direct access to the heart of government to protect their commercial interests and fight action on health at every turn.

The food manufacturers' lobbying group, the Food and Drink Federation, on its own had over 2,000 contacts with ministers, MPs, lords, MEPs, MSPs and special advisers last year.

"The lobbying is now intense. On the one side are all the health organisations you can think of, and on the other the vested interests in the industry. If the government takes no action, we will know the vested interests have won," said Debra Shipley, the Labour MP for Stourbridge who has been pushing for tighter controls on marketing to children.

The latest salvo in the campaign was a letter which arrived in Downing Street 10 days ago. The signatories included all the industry's big guns: Niall FitzGerald, president of the Advertising Association, also chairman of Unilever; John Sunderland, chairman of Cadbury Schweppes and newly appointed head of the CBI, who has signed twice, once as president of the FDF, and once again as president of ISBA, the voice of British advertisers, who spent over £452m on food advertising in 2002; David Felwick, chairman of the big supermarkets' lobbying group the British Retail Consortium, Tim Bennett, president of the National Farmers' Union, and Bob Cotton, chief executive of the lobby group for the catering industry, the British Hospitality Association.

The timing of the letter is not a coincidence. The report from the Commons health select committee tomor row will make some 70 far-reaching recommendations for urgent government intervention, acknowledging that the market can no longer be left to itself.

The MPs will recommend that the Food Standards Agency develop a traffic light labelling system to signal clearly which foods are bad for health. They are also expected to attack ministers' failure to come up with solutions to what they calculate is an astronomically expensive public health crisis.

The industry's letter suggests that in the face of the MPs' criticism, the government should focus on a voluntary "joint public health campaign", with "some sort of government endorsed symbol" which would promote "10 tips for better health". Instead of enforcing statutory labelling of food detrimental to health, the campaign would stress the importance of activity and of individual responsibility. People would be advised to "check out lower salt/sugar/fat choices" and to "try to drink more fluids".

The letter concludes that "we are at your disposition to discuss these ideas further ... and are copying this to the main secretaries of state at health, education, DCMS (culture, media and sport), and Defra (environment, food and rural affairs)".

The file of documents for the sports minister, Richard Caborn, gives a flavour of the lobbying the government has experienced. A letter to him from Cadbury Schweppes just after its managing director had been interrogated on obesity by the health select committee expresses concern that "the food industry is being viewed as part of the problem".

"Dear Richard ... I am enclosing ... some of the key messages ... which we will be trying to get across in the weeks to follow. We do hope that you can endorse these as a sensible way forward ..."

Mr Caborn had already given his public endorsement to another Cadbury's scheme for schoolchildren to collect chocolate wrappers in exchange for sports equipment. Every 5,400 chocolate bars would earn one volleyball set. A DCMS official trying to respond to the criticism found the normal channels had been bypassed. "We are trying to track down the source of the 'ministerial endorsement' that is being quoted back at us ... Is this something the minister did entirely off his own bat?" she wrote to the co-sponsor of the scheme, the Youth Sport Trust's chief executive, Sue Campbell. Ms Campbell is also a special adviser to DCMS and DfES.

Ms Campbell replied that the YST had agreed the endorsement of the Cadbury's scheme direct with Mr Caborn's private office. In fact the endorsement appears to have come from even higher up. A background note from another DCMS official records "there was a presentation to the prime minister, who was enthusiastic". Cadbury Schweppes said it did not make a formal presentation to the prime minister, but did make one to his special adviser, Sarah Hunter. The Food Standards Agency and the public health minister, Melanie Johnson, have both said recently that the Cadbury's school scheme should not have been allowed to go ahead.

Tessa Jowell, secretary of state for media, culture and sport, is the minister who will have to decide whether to restrict the marketing of foods to children. John Sunderland met her as representative of ISBA in February, but also separately as chairman of Cadbury Schweppes. He wrote to thank her on March 1. "FDF is seeing Dr Reid [secretary of state for health] on March 16 and will mention the idea of joint industry/interdepartmental discussions," he says. "I also, as FDF president, am dining with Mrs Beckett [secretary of state for environment, food and rural affairs] on March 10 and will seek her help in bringing this about. I shall take the liberty of mentioning that we are fortunate enough already to have your constructive support."

The briefing to Ms Jowell prepared by officials before her meeting with Mr Sunderland, wearing his advertising industry hat, suggests points she might like to make to him. These include that "it would be misleading to suggest that it is TV advertising that makes children fat", "there is no bad food, but there is a bad diet", and that "a review of TV food advertising is needed but the public debate is too focused on what children eat and not enough on how little physical activity children do".

Meanwhile over at the Department of Health, the FDF's director general, Sylvia Jay, had been in to put the food industry's case to the public health minister, Melanie Johnson. Ms Jay had written to congratulate Ms Johnson immediately on her appointment last June, and another letter four days later asked for a meeting. Officials advised it should be put off until the minister was better briefed.

When the meeting between Ms Johnson and Ms Jay did take place in January, it was to an agenda prepared by the FDF. "We have received a draft agenda from Sylvia Jay's office listing seven items and the meeting is only 45 minutes long," an official emails with a note of desperation.

Minutes of the meeting show that the FDF took the opportunity to tell the minister that the industry would oppose any proposals to reduce fat and sugar in foods along the lines of the work being done to reduce salt. It was also unhappy about the government use of logos, and opposed to fiscal measures to change dietary habits. However when it came to food in schools, Ms Jay wanted to help, and was keen to be on a steering group to look at the issues.

The FDF's own report to members summarises its achievements for 2003, name-checking many government departments and agencies. The highlights include:

· "Successfully argued against the adoption of EU legislation which could have undermined public confidence in the E-number system."

· "Helped prevent the adoption of EU flavourings legislation which would have imposed burdensome restrictions."

· "Achieved positive government response on the Employment Act. Despite strong trade union representation, there will be few changes ... Major role played by the FDF."

· "Achieved department of education endorsement" for industry material sent to schools."

Whatever the strength of tomorrow's select committee report, the food industry's lobbyists are turning their attention to a new and bigger battlefield: a Department of Health white paper on public health.

The FDF has held out the prospect of the white paper as the correct framework for discussions on regulation to take place, neatly bypassing the MPs' inquiry and the Consumers' Association's new campaign to name and shame junk food manufacturers. With consultation on the white paper not due to finish for another few weeks, the lobbyists will be hard at work for some time to come.