He conceded that he could be criticised for taking a cabinet post given that he has spent most of this parliament on the backbenches, but admitted he would find the offer of a government post more appealing than an opposition one.
In an interview with Alastair Campbell, Tony Blair’s former communications chief, for GQ magazine, Johnson also revealed that he had turned down a possible job offer from Miliband earlier this year.
Johnson, 64, has been the subject of speculation about a comeback since he quit as shadow chancellor in January 2011 because his marriage was ending.
He is acknowledged as one of the best communicators in the party and some Labour MPs were hoping he might replace Miliband until he used a Guardian article last month to rule himself out as a leadership candidate.
Johnson told GQ that Miliband had approached him earlier this year about the prospect of taking on a new frontline role in this parliament.
“Ed said to me a few months ago: ‘I suppose you’re not interested in coming back?’ I said ‘No’.”
Johnson, who held five cabinet posts under Blair and Gordon Brown, said that he had told Miliband he would be happy “to go round the country, campaigning, talking to parties” during the election. It was an election Labour could win, he said.
Asked about the prospect of taking a government post after a Labour victory, Johnson said that might appeal.
“Disgracefully, and it is disgraceful because I won’t have done the heavy lifting, then I would be more interested. But I am not gagging for it,” he said.
Johnson said he felt Labour should be doing a better job of challenging the Conservatives over the economy. Miliband should make “a couple of big speeches … just taking the whole thing apart,” he said.
“If I feel frustrated about anything it is about not holding George Osborne to account, because his flagship policy was to clear the deficit in one term. He has totally failed.”
Johnson also talked about how surprised he was to be offered the post of shadow chancellor when Miliband became Labour leader, not least because Ed Balls had made no secret of the fact that he coveted the role. But Johnson never seemed comfortable with the portfolio, and he told GQ he was relieved when he stood down.
“I was glad to get out,” Johnson said. “I took it because I thought, what an extraordinary gesture, that he wants me there. I was not associated with [Miliband], I’m very much a Blairite, I had not been planning to go on the frontbench. But my heart wasn’t in it. I was not up for it. I did it for months but I didn’t like the job.”
guardian.co.uk © Guardian News and Media Limited 2010