National Express operates rail and bus services
Richard Bowker, the chief executive of transport group National Express, is to resign, the BBC has learnt.
His resignation comes at a critical time for the firm, which is renegotiating terms for its rail operations with the government.
Its main rival First Group has also made a takeover approach.
National Express is due to release a trading statement later. Passenger numbers on its East Coast rail service have flagged during the recession.
National Express is contracted to pay the government £1.4bn to run the East Coast Mainline, which runs between Edinburgh and London, until 2015.
But sales at the franchise grew by just 0.3% in the first three months of 2009, compared with 9% over the whole of last year.
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Ministers say if National Express cannot make the franchise work it will have to give up all three of its rail companies - they have told the company there will be no bail-outs.
Cuts have been made in dividend payouts to shareholders and other spending while 750 jobs have been lost.
Last month, it started charging passengers for reserving a seat on its East Coast and East Anglia franchises.
The firm is currently trying to reduce a debt pile of about £1.2bn.