MP Richard Foord is calling for an election after James Heappey announced he will step down.
Mr Foord, the Liberal Democrat defence spokesman, said: “This is yet another blow to Rishi Sunak’s authority, as another Conservative minister abandons his failing Government.
“Sunak needs to put the country out of its misery and call an election now instead of allowing this farce to drag on any longer.”
Mr Heappey is set to leave his role as armed forces minister and quit as an MP.
The veteran has also confirmed in a letter to his constituents that he will stand down from the Commons at the next general election.
He said while it was a “painful decision” not to contest the election, he wanted to “prioritise my family and pursue a different career”.
Mr Heappey served in the Army, reaching the rank of major, according to a profile on his website.
During a 10-year career in the Rifles, he served in Afghanistan, Iraq, Northern Ireland and Kenya.
He had been hotly tipped to succeed Ben Wallace as defence secretary following his resignation last year.
But Mr Sunak instead gave the Cabinet post to Grant Shapps, who has no military experience.
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