Former Labour MP Oona King has announced she will run for mayor of London in 2012.
The former Bethnal Green and Bow MP will challenge former mayor Ken Livingstone, who will run again, to be named the Labour mayoral candidate.
The bid marks her return to politics after she lost her seat in Parliament to Respect's George Galloway in 2005.
Both the candidate for mayor and the party's next leader will be chosen at Labour's autumn conference.
Since losing her seat, Ms King, 43, has been widely involved in the media and is currently head of diversity at Channel 4.
Before she can take her challenge to the current Mayor of London Boris Johnson, she has to defeat her Labour colleague Mr Livingstone, who was mayor for eight years from 2000 to 2008.
Last year, Mr Livingstone, 63, said he would "love" to be mayor again and announced his own challenge to Mr Johnson, after the Conservative candidate beat him by 139,772 votes in the 2008 election.
Ms King said she would be giving more details about her campaign on Wednesday.