The need to reduce "carbon footprints" is persuading a growing number of travel management companies (TMCs) to switch to rail travel to London airports.
Kyle Haughton, sales and marketing director of the Airport Express Alliance which runs the Heathrow and Gatwick Expresses, said there had been a "quite significant acceleration" in business in the last 18 months.
He added: "We started at a small base but it has grown by 20% in that time."
Mr Haughton put the growth down to a need to cut costs and reduce CO2 emissions.
"A lot of companies are looking at the cost of transporting people around and one of the major costs here is getting people to and from airports.
"They want to reduce that cost which can be particularly high if you are based in London.
"But companies are also looking at their carbon footprints and how to reduce that while finance officers are keen to demonstrate what they are doing in the company accounts.”
"The consequence is that a lot of TMCs are working with the rail industry and Airport Express," he said.
Mr Haughton said he was "building up" relationship with UK TMCs which included Carlson Wagonlit Travel, Hillgate Travel, Portman Travel and Gray Dawes.
He said he was in talks with "several more" including one of the UK's largest agents.
Mr Haughton said Airport Express's business relationships with airlines was also "growing fast" but this was from a larger base.
Airlines with which the Alliance works include bmi, Virgin Atlantic, easyJet, JAL and United and it was in talks with American Airlines.