The capability of the fighting force is “well below what it should be”, Lord Richard Dannatt told Parliament.
The comments of the former head of the Army come as Prime Minister Boris Johnson is due to join NATO allies in Spain on Tuesday, against the backdrop of Russia’s invasion of Ukraine.
Under a major military overhaul announced last year, the number of main battle tanks will be slashed by a third and the number of soldiers reduced by 9,000, as part of an increased focus on drones and cyber warfare.
Speaking in Westminster, Government frontbencher Frederick Curzon, Earl Howe, stressed the importance of “defence capability rather than troop numbers”.
Under the Ministry of Defence’s Future Soldier plans, the Army would see the regular force strength cut to 73,000 personnel, down from 82,000.
Lord Howe said: “This will deliver a modern force that is more integrated, agile and lethal. Fit for the threats of the future and not the battles of the past.
“It will be better connected and faster, integrated across domains with allies in NATO and beyond.”
But Lord Dannatt said: “Would the minister not agree with me that the capability of the British Army is well below what it should be for a nation of our standing, a permanent member of the UN Security Council.
“In the days of the Cold War, not that long ago, we fielded four armoured divisions in Germany.
“We cannot field a single armoured division at the present time and there is a land war going on in Europe at the present moment.”
He added: “The situation is untenable and the Government must do something about this very soon.”
Lord Howe said: “I can assure him that under current plans the Army will be balanced to deliver right across the defence spectrum – to protect the homeland, engage with allies and partners overseas, constrain the aggressive activities of our adversaries and if necessary to fight wars.
“It is an Army that has been designed to fight but also organised to operate more productively and effectively.”
Former defence chief Lord Stirrup said: “Can he tell the House by what date this country will be able to field a full fighting division, with all the necessary attendant capabilities, including combat and logistic support and adequate weapon stocks?”
Responding, Lord Howe said: “He is well aware that we are not in a place that we would wish to be.”
The Future Soldier plan aims to deliver a modernised warfighting division by 2030.
Lord Howe added: “The Army is designed to fight. It will remain that way and we will ensure that it is equipped to do so.”
The British Army's capability is said to be 'well below what it should be' Corporal Rob Kane
General Lord Dannatt made the comments in Parliament Kirsty O'Connor