Food manufacturers have been pouring millions of pounds into a last-ditch attempt to block a European plan to put red warning labels on junk food.

In one of the biggest lobbying efforts ever seen in Brussels, lobbyists for Europe's £800m-a-year food industry have been bombarding MEPs with thousands of emails, letters and phone calls and sponsored reports, lectures and conferences ahead of a vote in the European Parliament tomorrow.

They are trying to prevent the European Union adopting the British Food Standards Agency's front of pack labelling system which displays red, amber and green "traffic lights" to indicate levels of salt, fat and other nutrients.

Although the scheme devised has the backing of doctors and dietitians, because it could cut Britain's annual toll of 70,000 diet-related deaths, the retailers Tesco and Morrisons and the multinational firms Nestlé, Kellogg's, Danone, Kraft, and PepsiCo have refused to introduce it. They back a rival scheme called Guideline Daily Amounts, which expresses the nutrients as percentages of an adult's recommended daily intake.

Independent research shows that traffic lights, which have the backing of the British Medical Association and the British Dietetic Association, are more effective than GDAs in putting consumers off unhealthy products. Despite that, in March the European Parliament's environment committee rejected traffic lights by 32-30 votes, following intense lobbying by manufacturers.

One MEP, Carl Schlyter, from Sweden, believes some MEPs shifted their views as a result of the pressure. "In the earlier discussions people were much more open-minded," he said. "But they have been exposed to so much industry pressure that it shifted focus."

A Dutch socialist MEP Kartika Liotard, who sat on the environment committee, said representations were 100-to-one in favour of the industry and during key meetings the room was so full of lobbyists that there were no seats for MEPs' assistants.

The lobbying has now shifted to the 736 MEPs who will vote on the adoption of a unified labelling system tomorrow. They will vote on three options: traffic lights with GDAs and the words "high", "medium" or "low"; GDAs based on percentages per 100g; or a calorie count.

Representations to MEPs by the food industry have been heavily skewed against the colour-coded scheme, claimed Glenis Willmott, the leader of Labour's MEPs.

Ms Willmott, who is also Labour's health spokeswoman, said: "Some food manufacturers... have poured enormous amounts of money, time and effort into challenging these ideas that would give consumers a better understanding of what's in their food.

"They don't want to see traffic-light labels because they don't want this kind of information in such an easy-to-understand format. They prefer complex labels that make it far harder for shoppers to really understand what's going in their basket."

In a report, Corporate Europe Observatory, a business watchdog based in Brussels, disclosed that Commission documents suggested that the PR and lobbying consultancy Fleishman-Hillard had been paid up to €671,000 to promote GDAs by the European confederation of food and drink industries, the CIAA.

"This has been a massive campaign from the food and drink industry, which clearly feels that the traffic light labelling scheme would damage its profitability," said Nina Holland, author of the report, A Red Light For Consumer Information.

The CIAA acknowledges its members spent €1bn on implementing the GDA scheme across Europe, but would not say how much they had spent on lobbying. In a statement the confederation, whose members include the UK Food and Drink Federation, rejected any suggestion its members had been exerting improper influence.

It said: "EU food and drink manufacturers have a legitimate interest in following this piece of EU legislation and we have made sure that European policy-makers are kept abreast of our views on this important draft law."

The vote is expected to be close. Among the groups expected to back traffic lights are the Greens and the centre left S&D which includes Labour, while the centre right EPP is mostly against.

The Eurosceptic ECR grouping including the Conservatives is also likely to vote against traffic lights. The Conservative group said it was too early to reveal the party's position.

Which?, formerly the Consumer's Association, says MEPs have the chance to back a simple system, traffic lights, that could explain what is in their food. Monique Goyens, director general of the European Consumers' Organisation, said: "Independent research tells us the colour-code labelling scheme, already used by some major supermarkets, is the system that shoppers find the most useful and easiest to understand."

The Food Information to Consumers Regulations would automatically become UK law, and would come into force in about three years' time. Labels would be mandatory for "complex processed food", such as ready meals, breakfast cereals, sandwiches, fizzy drinks, and "prepared products of animal origin" such as Cornish pasties.