Former chancellor Ken Clarke expressed "delight" at returning to frontline politics today - and insisted he would not rock the boat over Europe.
Mr Clarke said he had accepted the post of shadow business secretary, facing Lord Mandelson, because he wanted to help Britain tackle "the gravest economic crisis I have known in my lifetime".
"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 in a statement.
Mr Clarke - who stood unsuccessfully for the Tory leadership three times - said his pro-European views were "well-known".
"But I accept that the party has come to a settled view on European matters, and I will not oppose the direction David will set on European policies in the future," he added.
Amid claims that his appointment undermined the position of George Osborne, Mr Clarke stressed he wanted to work with the shadow chancellor "to rescue this country from the economic mess it is now in".
"I have thought carefully about returning to frontline politics," he said.
"I am doing so because this country faces a very serious situation - the gravest economic crisis I have known in my lifetime."
Mr Clark said Mr Cameron had returned the Tories to the "centre-ground of British politics", and offered a "credible alternative government for Britain, a real choice between a better future or more of the same under Labour".
"They have the right policies for dealing with the causes of the current economic crisis, and for restoring our economy to stability and growth."
Mr Cameron pointedly made clear that Mr Clarke would be "part of George Osborne's economic team".
"I am pleased and proud that Ken Clarke has agreed to join my Shadow Cabinet as part of George Osborne's economic team.
"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."
The shadow cabinet now combined "fresh thinking with experience, hope and change with stability and common sense", Mr Cameron added.
Mr Clarke - an acknowledged "big beast" of the Conservative Party - replaces Alan Duncan, who will be offered another post as part of a reshuffle today.
The appointment will pitch Mr Clarke directly against Lord Mandelson, whose surprise return to the Labour frontbench last year represented a similarly audacious gamble by Gordon Brown.
The announcement will delight supporters who see the former Cabinet minister as a genuine heavy-hitter who can add real weight to the Tory assault on the Government at a time when the economy is top of the political agenda.
However, others fear his strongly pro-European views risk reopening old wounds on the issue.
Only last week the veteran Euro-sceptic Lord Tebbit warned Mr Cameron not to bring back Mr Clarke, branding him "lazy".
There have been reports that major donors such as Lord Kalms and spread-betting tycoon Stuart Wheeler could withdraw support if he returns.
Mr Osborne today played down speculation that his position was reduced by Mr Clarke's comeback - which comes shortly after Mr Cameron anointed shadow foreign secretary William Hague as deputy leader "in all but name".
The shadow chancellor was seen to have been damaged by disclosures that he visited the Russian tycoon Oleg Deripaska on his yacht in Cyprus and was present when a possible donation to party funds was discussed.
But Mr Osborne told the BBC Radio 4 Today programme: "I actually was the person who first approached (Mr Clarke) and talked to him about getting more involved in the Shadow Cabinet.
"I think having him on board brings enormous expertise and experience, it strengthens the Conservative economic team and offers the country a real alternative government."
Mr Clarke's return was said to have been agreed at a lunch on Saturday at Mr Osborne's London home at which Mr Clarke, Mr Osborne and Mr Cameron were all present.
The 68-year-old has not served on the Tory frontbench since he mounted the first of his three unsuccessful leadership bids following Labour's landslide victory in the 1997 General Election.
He will bring with him immense experience, having entered Parliament in 1970 and begun his government career as junior whip two years later in Edward Heath's administration.
He went on to hold a series of major Cabinet posts, including Home Secretary, Health Secretary and Education Secretary.
Initially, the biggest advantage for Mr Cameron will be to add another instantly recognisable figure to a frontbench team which - William Hague apart - is painfully short of well-known faces.
Mr Clarke's blokeish bonhomie has made him popular with the public, although his bluff, bruising style has, at times, led to conflicts with colleagues.
Labour, meanwhile, will no doubt be quick to exploit his public support for a VAT cut as the quickest way to inject more cash into the ailing economy, despite Tory opposition to the move.