Cadbury's decision to move production of some of its chocolate bars abroad will lead to a massive increase in its use of diesel totally at odds with its environmental pledges, it was claimed this week.

The Transport and General Workers' Union said almost all of the products made at Cadbury's factory in Bristol, which is to close by 2010, were consumed in the UK and would have to be transported back here by road after production of them had been shifted to Poland.

The union said it had worked out with the help of Transport Watch UK that every truckload of chocolate made at the Polish plant, and transported back to the UK, would consume an extra 625 litres of diesel than if the bars had been made in England. It estimated that 15 lorries a day - or about 5,475 a year - would make the 18-hour trek, using an extra 3.4 million litres of diesel a year.

Earlier this year, Cadbury launched an ambitious environmental drive, dubbed Purple Goes Green, in which it pledged to slash its global CO2 emissions.

The company is also a member of the Food and Drink Federation, which last month published a series of green targets, including a pledge to cut food miles equivalent to running 350,000 cars on the road every year.

"British people consume approximately 98% of all products made at the Bristol site so they will have to be transported back to the UK," said TGWU regional officer Steve Preddy.

"We have presented Cadbury with these figures and it did not dispute them. We believe moving production to Poland will be ­detrimental to the environment."

Cadbury's nonetheless insisted it was confident it would still achieve its environmental targets and cut emissions overall.

"There will be a number of changes to the wider emissions picture as we move to fewer sites across the world but we are committed to achieving 50% net carbon emissions reduction by 2020 as promised," said Tony Bilsborough, Cadbury's media relations manager.

"That is the key measure on which we should be judged."