Alistair Darling has urged Jeremy Corbyn to make his policies clear if he wants "get off the ground", admitting he is confused about what the new leader stands for.
The former Labour chancellor, who stood down as MP at the last election, said the veteran left-winger has benefited from political disillusionment - but has failed to set out how he intends to make a difference.
In a swipe at Mr Corbyn's difficult first week as Labour leader, Mr Darling said he must recognise the importance of first impressions.
He told BBC Radio 4's Today show: "I know what he is against, but I'm not actually sure what he is for.
"What I would say if I had a conversation with him is he must know first impressions gained by the electorate, especially of a party leader, tend not to go away - and less than a week after he was elected he needs to spell out where he stands on key issues like the economy because if he doesn't, frankly he's never going to get off the ground.
"Actually, he is the leader and so far this week I just do not know where he is going and what he stands for.
"He really does need to frankly, in the next few days, set out with greater clarity what he wants to achieve and how he's going to do that."
Mr Darling, who said Mr Corbyn was his last choice for Labour leader, added: "There's a lot of disillusionment about, people are fed up the established order, they want change, Jeremy Corbyn benefited from that, people saying I don't want any of the above, I want something new. He's capitalised on that.
"If he really does say that he can make a difference, then he really must say how he's going to make a difference. I just haven't seen that yet."
Asked if he believed John McDonnell could become chancellor in government, he said: "It seems to me to be difficult but I'm willing to be surprised, I'm sure all clouds have a silver lining but I'm willing to be surprised."