Labour MP Bob Marshall-Andrews, who entered Parliament in the party's landslide victory in 1997, is to step down at the next general election.
Mr Marshall-Andrews said he "immensely enjoyed" being an MP
The serial rebel, who represents Medway in Kent, was a thorn in the side of former prime minister Tony Blair, notably over Iraq and civil liberties.
But he said Gordon Brown had got off to an "auspicious" start and hoped his regime would be less "authoritarian".
QC Mr Marshall-Andrews, 63, won a seat in the Commons at his third attempt.
Haemorrhage of votes
He has insisted that this announcement of his departure is final - after prematurely declaring himself defeated on the night of the 2005 election.
As the votes were being counted he appeared on TV and blamed the Iraq war for a "serious haemorrhage" of votes that had cost him his seat - then went on to hold it by 213 votes.
In a resignation letter on Tuesday to his local Labour Party, he admitted his time at Westminster had not been "entirely calm".
"Whilst I have supported the majority of the Government's programmes and initiatives (particularly in relation to the economy and public service), I have also strongly opposed other attempted legislation," he said.
"Overwhelmingly this has concerned civil liberty and, of course, Iraq."
He said he had "immensely enjoyed" his time as an MP.
"This has been made possible by the wonderful friendship and support, by the enduring goodwill of Medway's constituents (whatever their politics), by the assistance of our press and media (really) and by my outstanding staff who I will ultimately miss a great deal," he said.