Suspicions have been raised that BAA brokered a secret deal with the Government to allow a third runway at Heathrow after the air operator handed over £230m towards the Crossrail project.

Campaign groups Hacan and No- TRAG (No Third Runway Action Group) said the donation would cast doubt over whether the Government had promised to give the green light to expansion in return for the money.

Geraldine Nicholson, chairman of NoTRAG, said questions needed to be asked about what BAA would get in return, especially with the expansion decision looming.

She said: “£230m may only be a small part of the total cost of Crossrail, which is expected to reach £16bn, but it is money that a company like BAA with huge debts can ill-afford unless it has been promised something very good in return.”

John Stewart, chairman of Hacan, added: “The very fact that we are even asking these questions shows just how low the reputation of both BAA and the Department for Transport has sunk. Both organisations have lost the confidence of people at large.”

Crossrail would provide a link across the capital, connecting Heathrow and Maidenhead in the west with the City and Canary Wharf in the east.

Construction is set to begin in 2010, and services are due to start in 2017.

BAA denied the payment was part of an expansion deal with the Government, adding that Crossrail would take thousands of cars off the roads and complement its existing Heathrow Express service.

A spokesman said: “BAA is delighted to support the Crossrail scheme with a £230m funding package announced this week.

“This demonstrates our commitment to improving public transport access to Heathrow and reducing the environmental impact of our operation.

“It is entirely unrelated to the issue of airport capacity and in fact our discussions with the Department for Transport on Crossrail predate the aviation White Paper.”

The Department for Transport is due to announce its decision