Left-wing Labour MP John McDonnell has accused the party of "fixing" the race to succeed Gordon Brown as leader.
He said he wanted to stand but would struggle to get the backing of the 33 MPs he needed to get by next Thursday.
Jon Cruddas - who ruled himself out of the race on Monday - also said it was not long enough for MPs to make up their minds.
Only ex-ministers, and brothers, Ed and David Miliband have so far said they will stand for leader.
Former Schools Secretary Ed Balls and former Health Secretary Andy Burnham are also thought to be considering a bid.
http://news.bbc.co.uk/1/hi/uk_politics/8690259.stm