Conservative MP Stephen Dorrell, a cabinet minister in John Major's government, is to stand down next May after 35 years in parliament.
The MP for Charnwood in Leicestershire has accepted a job as a health policy consultant with accountancy firm KPMG.
Mr Dorrell, on the centre-left of the party, became an MP in 1979.
He was health secretary during the BSE crisis and a Treasury minister on "Black Wednesday", when the UK's pound crashed out of Europe's ERM.
As a backbencher, he served as chairman of the health select committee between 2010 and 2014.
In his resignation letter to the chair of his local party, Mr Dorrell said his decision marked a "bitter sweet moment".
His new position at KPMG would be "incompatible" with his role as an MP, he said.
He described himself as a "strong supporter of the Coalition" but pledged to campaign for a majority Conservative government at the general election.