Mr Cryer's Rover crashed at 1.24pm on the M1 near Junction 5 for Watford as he was driving down to London for the return of the Commons after the Easter recess.
His wife, Ann, was thought to have suffered minor injuries. Scotland yard said: 'The car overturned, ending up on its roof. It's believed no other vehicle was involved.'
John Smith, the Labour leader, said the whole party was 'shocked and stunned by Bob Cryer's tragic death'. 'He was an indefatigable socialist, a man of tenacious integrity, and a fine parliamentarian.
Mr Smith paid tribute to his 'genial wit and mocking sense of humour'. He said Mr Cryer, 59, a member of the Campaign Group, had been 'the House of Commons specialist on checking the abuse of secondary legislation and he was a terrier-like upholder of the rights of Parliament'.
Mr Cryer's death means a by-election in a safe Labour seat. His majority was 4,902 over the Tories in the 1992 election.
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