Gen Dannatt's new role was announced at the Tory conference
A former head of the civil service has called the appointment of ex-Army chief Sir Richard Dannatt as a Tory defence adviser "a major error of judgement".
Lord Turnbull, who from 2002-2005 was cabinet secretary, was giving evidence to a House of Commons committee.
It could subvert the chain of command and see an Army man deciding on cuts across all three services, he said.
Lord Turnbull told the MPs: "You talk to admirals and they are incandescent about this."
He thought the biggest objection to the general's appointment was that it "cast a shadow" over his successors because people would start wondering about their political affiliations.
And if he were to become a defence minister in the future "where did it leave the position of the new chief of the general staff if his predecessor is in the ministerial team?"
The Labour chairman of the Commons Public Administration Committee, Tony Wright, asked Lord Turnbull if he thought the error was on the part of General Dannatt or Conservative leader David Cameron.
Lord Turnbull replied: "I'll leave you to judge that."
Several Labour MPs have expressed concern that General Dannatt should have allied himself with the Conservatives so soon after he stood down as head of the army and while he is still being paid by the military.
The Conservatives insist they did not have any discussions with him about a future advisory role while he was still in the army and say his expertise will be vital for them should they form the next government.
General Dannatt, who will be nominated for a peerage by the Tories, is not the first member of the armed forces to move into politics.
Admiral Lord West has been a security minister in the Labour government since 2007, although in his case he took up the job one year and four months after leaving the forces.