A failure by the Conservatives to commit 2 per cent of national income to defence over the next parliament risks losing the support of military voters to Ukip, defence sources warn.

Nigel Farage this week made a pledge to keep British defence spending above the minimum Nato threshold of 2 per cent of GDP — a hugely popular move for members of the armed forces and something that none of the major parties is expected to do.

One former army officer, a lifelong Tory, said he was “thinking hard” for the first time about defecting to Ukip after Michael Fallon, the defence secretary, refused to guarantee an enduring commitment to 2 per cent beyond the current financial year.

“I am really disappointed,” the officer said,