Boris Johnson has failed to guarantee troop numbers will be spared cuts as he unveiled a major review of Britain’s foreign policy.

The Prime Minister was pressed by Labour to explain whether the number of frontline personnel will fall, amid reports the Army could reduce by 10,000 in the coming years.

After Conservative Richard Drax (South Dorset) suggested boosting the Army’s regular force beyond 100,000, Mr Johnson said the combined strength including reservists will remain above 100,000 but “tough decisions” will be required.

In 2015, the then government’s review committed to maintain the size of the regular Armed Forces and to not reduce the Army below 82,000 troops while also making increases to the Royal Navy and Royal Air Force.

In recent years the Government has been working to boost a shrunken regular force of 82,000 with an increased reserve force of 30,000.

Speaking in the Commons, former Army officer Mr Drax asked: “Global Britain remains the buzzword and our armed forces have a key role to play with the Integrated Review and with that in mind, and the many responsibilities (Mr Johnson) has committed to, can he reassure me that he will expand the Army to 100,000 as our chief ally the US has recommended rather than see it wither on the vine to 72,000 by cutting recruiting – thereby avoiding redundancies?”

The Prime Minister replied: “The Army, including reserves, (Mr Drax) will be pleased to know, will be over 100,000.

“But it is the duty of this Government to take the tough decisions that are necessary to modernise our armed forces as well.”

Labour leader Sir Keir Starmer earlier said: “The Prime Minister can’t avoid the question that everyone in our armed forces and their families will be asking today: will there be further cuts to the strength of our Army and our armed forces?

“The British Army is already 6,000 below the minimal level set out in the last review, it’s been cut every year for the last decade, and it’s been reported that the Army will see a further reduction of 10,000 alongside fewer tanks, fewer jets for the RAF and fewer frigates for the Royal Navy.”

Sir Keir went on: “Successive Conservative prime ministers have cut the armed forces, but at least they have had the courage to come to this House and say so.”

Mr Johnson said Labour had the chance to back the armed forces in the Overseas Operations Bill but voted against it.

He said: “And they want to talk about standing up for our armed forces, just in the last year the Labour Party has been given the opportunity to back our armed services, our armed forces, our troops, our soldiers in the Overseas Operations Bill.

“They had the chance to stand up for veterans, they voted against it on a three-line whip.

“Those are the instincts of the Labour Party: weak on supporting our troops, weak on backing Britain when it matters and weak on defence.”

Labour’s Nick Smith (Blaenau Gwent) later asked: “Will the Prime Minister level with the country and tell us exactly how he plans to reduce the armed forces by 10,000 in the years ahead?”

Mr Johnson replied: “I’ve already explained that this is the biggest investment in our armed forces since the Cold War and there’ll be no redundancies across the armed forces.”