Sarah Teather, the Liberal Democrat MP for Brent, has announced she will quit Parliament at the 2015 election, citing disappointment with recent party policies.
She has been the leader of Brent since 2003, when at age 29 her election made her the youngest MP in Britain. She also served as children's minister in the coalition government before returning to the backbenches last year.
In a statement on her website, she said: "I have disagreed with both government and official party lines on a whole range of welfare and immigration policies, and those differences have been getting larger rather than smaller."
One of the main reasons she pointed to was the Lib Dem's decision to back the Tories' planned cap on welfare while she was in Government. She also said that Lib Dem leader Nick Clegg's plan to make some immigrants to pay a £1,000 deposit when applying for visas left her feeling "utterly desolate"
A Lib Dem spokesman said it was "disappointed" by herdecision - which comes a week before the annual party conference - and defendedits "proud record" in the coalition Government.