Former Army head falls in with Tories
But General Sir Richard Dannatt's new role confused the shadow home secretary.
Sir Richard - who has been embroiled in a bitter row with Downing Street over troop levels in Afghanistan - is expected to become an adviser to the party on defence issues.
Mr Cameron's spokeswoman refused to give details of the appointment, which is due to be included in his speech to the Tory conference in Manchester.
But she said: "I can confirm that there is an announcement tomorrow."
Sir Richard is set to be made a peer as part of the deal, meaning he could serve as a minister.
Speculation has been mounting over the former Chief of the General Staff's future role.
He fuelled the rumours in a round of interviews this morning by saying he would "theoretically" be interested in joining Government."I have always taken the view that I will get on with the job at hand and do it as well as I can," he said.
"If someone in a future life wants me to do something else, well, let them ask the question and I will give the answer."
He went on: "No-one's asked me that question - that question has not been put in a public way."
Speaking at the Conservative conference in Manchester, Mr Cameron said: "I have spoken to him on previous occasions, as well as recently, about his views about how we improve our armed services and support their families and make sure we rebuild that military covenant, how we successfully pursue what we are doing in Afghanistan.
"He is a man of great talent and ability. He has been a great public servant and I think he has more to do."
Sir Richard assumed the ceremonial post of Constable of the Tower of London today, having stepped down from the army over the summer.
This week he confirmed that Gordon Brown had previously refused requests for troop levels in Afghanistan to be boosted by 2,000, saying the Prime Minister had viewed the move as "too expensive".
Shadow home secretary Chris Grayling said he hoped General Dannatt's appointment was not a "gimmick" - apparently under the mistaken impression that it was Gordon Brown, and not his own leader, who was offering the general a job.
Mr Grayling told BBC News: "I admire the work of General Dannatt and other senior generals who've done so much in Afghanistan and done so much to lead.
"I hope that this isn't a political gimmick. We've seen too many appointments in this Government of external people where it's all been about Gordon Brown's PR.
"General Dannatt's an experienced figure and should rightly be working alongside government. I'm always suspicious of Government's motives when it does things like this."
Home Secretary Alan Johnson seized on the gaffe, saying: "What we've seen today is that Chris Grayling is so keen to do Britain down, he'll attack anything - including his own party."
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