The authors of a government-commissioned study into 60-tonne superlorries have come under pressure to alter their findings to give ministers an excuse for rejecting road trials.

Ruth Kelly, the Transport Secretary, has repeatedly broken commitments to publish the study, carried out by Heriot-Watt University and the Transport Research Laboratory. It was submitted to the Department for Transport (DfT) nine months ago.

The Times revealed last November that the study had found compelling evidence that extending the maximum permitted length for a lorry from 61ft to 83ft – 24ft longer than a bendy bus – would reduce carbon emissions and industry costs without compromising safety.

The Government promised in November to publish the study by the middle of February. The deadline was then put back to the end of February but that date was also missed. Two weeks ago Ms Kelly said that it “should be published before the summer recess”.

A source close to the study said part of the reason for the delay was that the DfT had been trying to persuade the authors to make their findings less positive. The source said that the authors had refused to allow their independence to be compromised. “You have to draw a line between accepting amendments to clarify arguments and changing the final conclusions and recommendations.”

The study found that longer, heavier lorries, known as LHVs, would reduce the overall number of articulated vehicles: two could do the job of three existing lorries.

While the weight carried would increase by only 30 per cent, the volume would rise by 60 per cent. This would particularly benefit companies that move light but bulky goods.

Rail freight companies have joined forces with rail unions to put intense pressure on the Government to reject LHVs without carrying out road trials.

Ms Kelly appears to be preparing to rule out trials on the ground that superlorries would do little to improve congestion.

She told the House of Commons last month: “I would need a huge amount of persuasion that superlorries are the answer to congestion on our roads.”

Long loads abroad

— Sweden and Finland have permitted LHVs since the 1960s

— The Netherlands approved them last year after finding that they slightly reduced road deaths by cutting lorry mileage

— They have been tested in some parts of Germany

— Denmark and Belgium are carrying out trials this year