Government critic Alan Simpson is to stand down as a Labour MP at the next general election.
Alan Simpson has been critical of Tony Blair's premiership
A long-term critic of Tony Blair over the invasion of Iraq, Mr Simpson has also opposed the prime minister's support for new nuclear power plants.
In a letter to his constituency party members, the MP said a Gordon Brown-led premiership would be even worse.
Mr Simpson, 58 and first elected for Nottingham South in 1992, said there were "not enough" good Labour MPs.
He said he would be more effective campaigning for radical environmental change outside parliament, rather than remaining on the back benches.
Mr Simpson said in a letter to party members: "There are good people in the Parliamentary Labour Party but not enough of them. At times, I feel that colleagues would vote for the slaughter of the first born if asked to."
He added that another factor behind his decision is the birth last year of a baby daughter from his third marriage.