EasyJet has taken Virgin Trains to the advertising watchdog in a fight over which mode of transport has the strongest green credentials.

The no-frills airline has complained to the Advertising Standards Authority about a Virgin marketing campaign which claims that a train journey emits three-quarters less carbon dioxide than a comparable trip by air. EasyJet also alleges that Virgin had ramped up its passenger numbers to flatter some figures and failed to disclose whether its trains use electricity from nuclear power stations.

An easyJet spokesman said the environment was becoming a new battleground for the travel market, amid indications that climate change could become as big a factor in consumers' considerations as price. "We believe this is a very important topic, because we are going to be seeing more and more adverts on the environmental issue," he said.

EasyJet has been caught out by the same issues already this year. The ASA ruled last month that the airline had "inaccurately portrayed" the environmental benefits of its new fleet of planes.

A Virgin Train spokesman said the research in its advert, carried out by the Edinburgh Centre of Carbon Management, had been sent to the ASA already and the company had no intention of pulling its campaign. "This really smacks of desperation. If an airline really thinks it can challenge a train company on these figures it is barking up the wrong tree. We stand by these figures and we will continue to advertise them." The spokesman added that Virgin bought its electricity from Network Rail and it had asked the rail infrastructure company to give more details on what proportion of its power is generated by nuclear facilities.

"We are asking for more detailed figures from Network Rail. We need better knowledge of how much of our energy is taken from nuclear power stations," he said.

Airlines account for 5.5% of all CO2 generated in Britain, which is dwarfed by the contribution of road transport and energy companies, but is one of the fastest growing sources of carbon emissions.

Earlier this year the environment minister, Ian Pearson, labelled Ryanair, Europe's largest no-frills airline, the "irresponsible face of capitalism" over its stance on the environment and its refusal to join the EU's emissions trading scheme. The industry has so far failed to mount a co-ordinated response to criticisms from green campaigners and politicians, amid infighting over who should lead an industry body to rebut environmentalists' claims. British aviation companies have formed a new green association, but tensions between British Airways and Virgin Atlantic have delayed its launch.

In another dispute over advertising, Friends of the Earth has complained about a Shell advert, showing a Shell refinery spewing out flowers rather than smoke, which suggests that the oil group uses all of its waste CO2 to help grow flowers. FoE claims that less than 0.5% of the carbon output is recycled in such a way.