Speculation is growing over a possible deal between the Government and BAA for a third runway at Heathrow Airport after the operator pumped cash into the Crossrail project.

It was revealed on Tuesday that BAA invested £230m into the £16bn rail scheme, which was given the go-ahead in July this year.

Although businesses who are set to benefit from Crossrail are injecting money into the Government-funded project, there are suspicions among campaigners against further expansion at Heathrow Airport about BAA's involvement.

When up and running, Crossrail will link Maidenhead to Shenfield in Essex, with stops in central London and Buckinghamshire.

A connection will be made to Heathrow Airport and will replace the exisitng Heathrow Connect service which runs from Terminal four to London Paddington.

BAA's obvious gain and reason for investing is the improved link for passengers to the airport. However, campaigners such as the No Third Runway Action Group (NoTRAG) and HACAN are speculating whether the cash injection is a way of "buying" a third runway.

They are speculating whether in return for the money, the Government would then approve plans for the expansion next month.

Geraldine Nicholson, chairman of NoTRAG, said: "At the very least questions need to be asked what payback BAA may be getting in return for this donation.

"£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 McDonnell, MP for Hayes and Harlington, said although his constituency will gain from Crossrail, because one of the stops will be in the area, he branded the investment a way of influencing the Government to give the green light to expansion.

Mr McDonnell said: "I have been in support of Crossrail from the start, because it will put the Hayes and Harlington station on the map and link Hayes to London within 20 minutes.

"It is going to provide massive job opportunites and enhance the character of Hayes. Businesses are already using Crossrail to promote what they do.

But he added: “BAA are trying to use their investment on Government to buying their way into the third runway.

“BAA’s investment in Crossrail should not be used as a way of influencing the Government to allow the third runway."

Stuart Butchers, spokesman for BAA, denied the claims that the operator is trying to buy a third runway.

He said the scheme "demonstrates commitment to improving public transport access to Heathrow."

He said: "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 in 2003."

The white paper aims at developing airport capacity over the next 30 years