Gordon Brown considered at least three other candidates before appointing Bob Ainsworth as Defence Secretary last month.

The Prime Minister came within an hour of appointing Jim Knight, then the Schools Minister, before handing the job to Mr Ainsworth to appease a faction of the Labour Party.

Doubts have grown about whether Mr Ainsworth, one of the most junior members of the Cabinet, has the authority to marshal support for the military, either within Whitehall or among the wider public.

The manner of the appointment also encapsulates Mr Brown’s struggle to retain authority over his party in the febrile atmosphere after Labour’s disastrous showing in June’s local and European elections.

Mr Ainsworth’s predecessor, John Hutton, had indicated to Mr Brown in mid-May that he was thinking of leaving the Government. Mr Hutton, recently remarried, had a compelling family reason for wanting to step down. But Mr Brown, preoccupied with the elections and the possibility of a leadership challenge, appears to have spent little time thinking about the vacancy. It wasn’t until around noon the day after the polls that he began to focus on who should oversee Britain’s military and its engagement in Afghanistan.

In the midst of a reshuffle carried out against the backdrop of political crisis, a weakened Prime Minister had fewer candidates with Cabinet experience willing to serve and more vacancies than he had expected. Although the field for Defence Secretary was crowded, only one was a serving Cabinet member. Shaun Woodward, the Northern Ireland Secretary, had made little secret of his desire for the job. John Spellar, a senior whip and former Armed Forces Minister, was another name pressed on Mr Brown. However, it was Mr Knight who emerged as a front-runner. His candidature became so well advanced that the appointment came within an hour of being announced, according to one senior figure.

None of the three candidates, however, delivered much political capital to Mr Brown as he fought for his survival. In this fevered atmosphere it was the departure of the Farming Minister that proved decisive. Jane Kennedy’s resignation prompted fears among Mr Brown’s advisers that he was losing the support of a faction of Labour MPs to which she belongs. Generally on the right of the Parliamentary Labour Party, the group of MPs includes Alistair Darling, Geoff Hoon — and Mr Ainsworth.

Mr Ainsworth, then Armed Forces Minister, had been in the running from the off. It is likely that he was Mr Hutton’s recommendation as he offered continuity. Ms Kennedy’s departure and Mr Brown’s weakened position meant that the Prime Minister was no longer in a position to resist an appointment that would satisfy both defence chiefs and Labour MPs.

“Bob is liked by the military,” a friend said. “They like his toughness. They like his occasional laddishness.”

Another said that the former trade union official had fallen victim to “snobbishness” in the media. “This feeling that Bob is somehow an NCO and not officer material is unfair.”

Concern may have been fuelled by Mr Ainsworth’s ranking in the Cabinet where he is placed at 21 out of 23. However, this just ranks all but the most senior seven Cabinet ministers according to length of service. Nevertheless, the Defence Secretary knows he faces a fight to convince voters that, whatever his official ranking, he is the best man for the job.

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