Former Culture Secretary Tessa Jowell has announced she is standing down as a Labour MP.
Ms Jowell, who was one of the key figures behind the London bid for the 2012 Olympics, said the time was "right" to leave Parliament after the next general election, in 2015.
She told party members in her Dulwich and West Norwood constituency her career had been a "privilege".
Former Prime Minister Tony Blair said Ms Jowell had achieved "great things".
Ms Jowell, who was first elected to Parliament in 1992, told Labour activists in Dulwich and West Norwood: "This is, I think, the hardest decision I have ever taken."
She added that, since selection as a candidate in 1990, she had "felt the strongest bond with, and commitment to, our constituency" and that she had "loved every day that I have served as your Member of Parliament".
Ms Jowell, who was known during her time in government as one of Mr Blair's most ardent supporters, told Labour leader Ed Miliband of her decision last week.
In his letter of reply, he wrote: "You are a unique politician. Passionate about the people you represent, deeply loyal to your party, generous to all you meet, from whatever walk of life, with a warmth and a spirit that anyone who has met you will immediately recognise."
Mr Blair said: "Tessa Jowell is a very unusual type in the often brutal world of politics. She is immensely able, tough-minded and determined. But at the same time and with the same people, she is kind, decent and loyal in a way I have seldom seen in politics.
"I always knew I could rely on her and trust her 100%. But I also knew she would never hesitate to tell me what she thought was right and true. That gave and gives her a rare integrity."
She was culture secretary from 2001 to 2007 and later served as Olympics minister and paymaster general.
Lord Coe, former chairman of the British Olympic Association, said: "I'm very sorry that Tessa has decided to step down from constituency life. It goes without saying that both her constituents and parliamentarians from all sides of the House will miss her.
"She was the political driving force behind our bid to host the London Olympic and Paralympic Games and an inseparable part of their ultimate success. I will be forever grateful for the role she played in both."