The Prime Minister came under the most intense pressure yet to allow expansion at Heathrow when Brendan Barber, the general secretary of the TUC, wrote to him committing the union movement to the project preferring it to the proposed development of Stansted in Essex.
It is understood that Mr Barber is seeking an early meeting with Tony Blair to emphasise the importance of a new short runway at Heathrow.
The TUC's letter, obtained by The Independent on Sunday, calls for three new runways in the south-east.
Until now, the union movement has argued in favour of increased capacity, but kept out of the controversy over Heathrow.
Mr Barber says the Government has calculated that £7.8bn worth of economic benefits and nearly 100,000 jobs would be generated by the expansion of Heathrow.
"The Government should be prepared to strain every sinew to work with the industry to ensure that expansion can be achieved whilst meeting the air quality standards," he wrote.
Dan Hodges, the director of Freedom to Fly, which campaigns for more airport capacity, said the letter underlined the importance of expansion not only for the companies involved in aviation but for working families who wanted the extra jobs it would create. "Without the extra capacity at Heathrow ordinary people would also find air travel impossibly expensive."Reuse content