By Alex Stevenson

A gaffe from Chris Grayling has ruined the potential unveiling of former Army chief General Sir Richard Dannatt as a future defence minister.

The shadow home secretary, who was not aware of the impending announcement and appeared to assume he would enter a Labour rather than a Conservative government, told the BBC he suspected the move was a “political gimmick”.

Gen Dannatt’s outspoken comments against the government have proved consistently embarrassing to ministers in the Ministry of Defence.

He revealed Gordon Brown refused to increase the number of troops serving in Afghanistan despite his demands and is now widely believed to be unveiled by David Cameron tomorrow as a potential future member of a Tory government.

Mr Grayling said: “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 he does things like this.”

In a separate interview, Gen Dannatt told the BBC the government had sought to undermine him personally as he continued to vociferously demand more support for Britain’s troops.

Ministers resorted to “politics by smear”, he said.