George Galloway is to run for the London Assembly in May's elections as part of a "progressive list".
George Galloway is renowned for his outspoken political comments
He promised the group would gather a "broad coalition" of left-wingers who would promote "Old Labour values".
Mr Galloway, MP for Bethnal Green and Bow, has taken the decision amid splits within the anti-war Respect coalition.
John Rees, the coalition's general secretary, said he was sure the party would still "do very well without him". Mr Galloway denied he had left Respect.
Respect saw a split in its ranks last year, holding two separate party conferences on one day last November.
The division was understood to revolve around arguments about candidate selection and internal democracy.
Mr Galloway and his supporters stuck together on one side, with representatives of the Socialist Workers' Party on the other.
As well as standing as part of a progressive list - which Mr Galloway says will include some Respect members as well as people not part of Respect - Mr Galloway still plans to challenge Transport Minister Jim Fitzpatrick in his Poplar and Limehouse seat.
If he does participate in that east London constituency at the next general election, he will be hoping to claim his second Labour scalp.
In the 2005 election, he spectacularly defeated Oona King, overturning the Labour MP's majority of 10,000 to win by 823 votes.