Former chancellor Ken Clarke has expressed "delight" at rejoining the Conservatives' front bench team, as shadow business secretary.
He said he had thought carefully about returning to frontline politics and decided to do so to tackle "the gravest economic crisis I have known".
The pro-European said he accepted the party had a "settled view" on the issue and he would not oppose its policy.
David Cameron said Mr Clarke, 68, was a "big figure" with "great experience".
"Ken was the last chancellor of the exchequer to lead this country out of recession. He has more experience of dealing with tough economic challenges than Gordon Brown's entire cabinet," Mr Cameron said.
In other reshuffle moves, Chris Grayling becomes shadow home secretary, Eric Pickles becomes party chairman and Theresa May becomes shadow work and pensions secretary. Alan Duncan, who had been covering the business brief, will replace Ms May as shadow Commons leader.
And the position of shadow Europe minister, retained by eurosceptic MP Mark Francois, has been elevated to a shadow cabinet post.
Mr Clarke served as health, education and home secretaries, as well as chancellor, between 1988 and 1997.
It's good to have someone in the shadow cabinet who has supported our policy on Europe
But his pro-European views, in particular his support for Britain joining the Euro, put him at odds with many in his party and are thought to have cost him the leadership in contests in 1997, 2001 and 2005.
Mr Clarke said he had thought hard about whether to return to the Conservative front bench, adding in a statement: "I am doing so because this country faces a very serious situation - the gravest economic crisis I have known in my lifetime."
He said under David Cameron's leadership, the party had returned to the "centre ground of British politics" and offered a "credible alternative government".
"It is going to be a historically important election, and I don't want to sit on the sidelines - I want to be out on the pitch fighting for the change Britain needs," he said.
He said his pro-European views were "well known" but said he would not oppose the direction set by Mr Cameron on European policy.
Shadow chancellor George Osborne denied suggestions he would be overshadowed by Mr Clarke, telling BBC Radio 4's Today programme: "I've been talking to him pretty constantly for the last year, I actually was the person who first approached him and talked to him about getting more fully involved in the shadow cabinet."
Rushcliffe MP Mr Clarke, known for his love of cigars, jazz and classic cars, is thought to have popular appeal outside Westminster.
In his new role he will shadow Lord Mandelson, who returned to the cabinet in the October reshuffle and who welcomed Mr Clarke to the role saying he was "an old political friend".
Mr Clarke opposed the Iraq war, which was supported by the Conservative front bench, and also suggested cutting VAT to 15% before the government announced it in November's pre-budget report.
However, he later criticised the circumstances in which the government implemented the change, insisting that it could not fund the move, which has been branded "a criminal waste of money" by Mr Cameron.
David Cameron and I have never had any problems at all on any other subject and we just know we disagree on Europe
Asked about Mr Clarke's return, Prime Minister Gordon Brown said: "It's good to have someone in the shadow cabinet who has supported our policy on Europe, supported our policy on VAT and is probably quietly supportive of many of our other policies."
But speaking outside his home, Mr Clarke told reporters: "Peter Mandelson is much more pro-European than Gordon Brown is but in Peter's case, that's just the first part of a whole history of political and personal bad blood between them, that goes back years.
"David Cameron and I have never had any problems at all on any other subject and we just know we disagree on Europe. I know that the party's not going to change its policy and I'm not going to be trying to get the party to change its policy."
'Out of line'
However the eurosceptic UK Independence Party claimed his appointment had already led one party member, a town councillor, to defect to them.
And Barry Legg, of the Conservative eurosceptic Bruges Group said it effectively meant "the end of David Cameron's promise to hold a referendum on the EU Constitution".
"The so-called 'big beast' has trampled all over Mr Cameron as the price of his return to the Conservative front bench," he said.
But Tory chairman Mr Pickles told BBC Radio 4's World at One: "Ken is a big beast by any standard and he's there to get us elected as a government...
"We don't want Ken to be quiet. We want Ken to be on television all the time. We want him to be putting out our ideas."
Former Conservative cabinet minister Michael Portillo said Mr Clarke's return was a "justifiable" risk for Mr Cameron.
He added that a return to the front bench for former shadow home secretary David Davis would "strengthen the Conservative team once more".
Other reshuffle moves include Dominic Grieve moving from shadow home secretary to shadow justice secretary. He replaces Nick Herbert, who will take on the shadow environment brief.
Changes among junior shadow ministers include Richard Benyon becoming a shadow environment minister, Crispin Blunt being made shadow security minister and David Evennett moving to innovation and skills.
Justine Greening moved to local government, Greg Hands became a shadow Treasury minister and John Penrose moved to be a shadow business minister. Bill Wiggin and Robert Wilson moved to the Whips' Office.
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