Labour's shadow cabinet will "pull together" and leave behind the problems of the past following Alan Johnson's decision to quit frontline politics, Douglas Alexander has said.
The shadow foreign secretary said Mr Johnson, who resigned on Thursday, had been a "significant" figure and would be a loss to the party.
Mr Alexander was moved to the foreign affairs role in a mini-reshuffle forced by Mr Johnson's departure. He said Labour had to act as a "credible alternative" to the coalition.
Mr Johnson quit as shadow chancellor citing personal issues amid allegations that his close protection officer when he was Home Secretary, Pc Paul Rice, had an affair with his wife.
Labour leader Ed Miliband then installed Ed Balls in the key role after overlooking him last October.
Mr Alexander said: "I don't think that anybody can deny that the loss from the frontbench of as significant a figure as Alan Johnson is a loss for any political party.
"But there is a new team in place. Ed has made his decision in terms of the personnel and I think we are determined to pull together, leave behind some of the problems of the past and start anticipating some of the questions that Labour needs to be answering that people will be asking us over the next three, four or five years."
He added: "The real challenge for Labour is not just to express anger but to offer answers, not just to be a fighting opposition but a credible alternative. That was true last week, it remains true this week."
Put to him on BBC1's Andrew Marr show that the Labour Party was moving to the left, Mr Alexander said: "What lessons do all of us of this generation draw about the past? Certainly that the era of Blair and Brown is over but also that era of division and of factionalism did the Labour party a lot of damage.
"The lessons that I actually draw is that we do need to be economically credible in order to be electorally viable. We do need to pull together as a team and we need to be absolutely clear that we are appealing to broad support right across the United Kingdom."
Reminded that Mr Balls, a fierce ally of former prime minister Gordon Brown, has questioned Labour's deficit reduction plans in the past, Mr Alexander said: "He said on the first day in office as shadow chancellor that he accepts the position that Ed set out in this studio last week that if we were in government we would be halving the deficit over four years.
"He has also been very clear in the newspapers this morning that he genuinely thinks, and I completely agree with him, that George Osborne is making choices that will mean a longer dole queue, higher welfare bills and actually give us more difficult circumstances to deal with."
And he said Labour would follow the economic position set out by former Chancellor Alistair Darling: "That remains the position."
Mr Alexander said the Liberal Democrats were now "locked into a project and a narrative of the centre right".
He added: "Our job certainly is with an open heart and an open mind to work with them, to try and make good some of the bad decisions the coalition Government is taking, but on the other hand to hold them to account at the ballot box for the fact that the electoral logic for the Liberal Democrats now is pointing towards a centre-right future for Britain not a centre-left future for Britain."
Labour's deputy leader Harriet Harman insisted the shadow cabinet was united behind its economic policy and said Mr Balls and Mr Miliband were in "strong agreement".
She told the Murnaghan programme on Sky News: "We are all agreed on this, that the line of argument was established following on from what Alistair Darling did while in government. The line was established by Ed Miliband and Alan Johnson and that is the line we are taking forward with Ed Balls as shadow chancellor after Alan Johnson's very sad resignation."
She added: "We think it is right to reduce the deficit by half over four years and to go faster than that or further than that is to take a risk with the economy."
Speaking about Mr Johnson's resignation, she said: "I am very disappointed indeed that he has felt that he needed to resign because I think that is a loss not just to the Labour party but to frontline politics.
"He rose to the highest offices in the land but he really never lost touch with his roots and I think he was an exceptional member of our frontbench team.
"I am very sorry to see him go but I think that Ed Balls will be a very strong appointment as part of our team."
She added: "I think he will be very effective. He is an experienced economist."