Former London MP Oona King will announce tomorrow that she will take on Ken Livingstone to be Labour's candidate for Mayor.
"King will fight Livingstone to become Labour challenger to Boris Johnson. The contest – like the leadership contest – will be decided at the party conference in the autumn."
Oona will officially launch her campaign on Wednesday.
According to Guardian writer Jackie Ashley, Oona thinks that Ken was a "good Mayor" but counters that:
"It's nobody's birthright, and I don't believe in the hereditary principle."
Oona left frontline politics in 2005 when she lost her seat to George Galloway.
A fan of Gordon Brown, Oona was tipped for a job at Number 10, but was dismissed as a "lightweight" by some.
No other candidates have declared so far, although a few including former Home Secretary Alan Johnson have been mooted in recent months.
Since her announcement King has moved up to fourth place at 12/1 to be the next Mayor behind Alan Johnson, Ken and Boris.
More details about the race are expected to be revealed this coming week with nominations closing in June.