Labour backbencher Jon Cruddas has announced he will not be standing against the Miliband brothers for the party leadership.
The Dagenham MP, who has strong union backing and finished third in the 2007 deputy leadership contest, said "many people" had urged him to stand and he had given it "serious thought".
But he added: "I do not feel that I am in a position to deliver on the hopes and expectations that will be placed in the next leader."
Only David Miliband, his brother Ed Miliband and veteran left-winger John McDonnell have so far declared their candidacies in the race to succeed Gordon Brown.
Other senior Labour figures Harriet Harman and Jack Straw have ruled themselves out. Ed Balls and Andy Burnham are biding their time before making an announcement.
Mr Cruddas, a link man between Number 10 and the unions during Tony Blair's premiership, is being tipped for the role of party chairman after successful organising the campaign against the BNP in his east London constituency.