Britain’s first ground force in Ukraine would be reinforced by at least 1,000 reservists, under current proposals drawn up by Army planners. Provisional plans drawn up at Army Headquarters in Andover have set out the need for part-time soldiers to fill infantry, engineering and medical roles if the Prime Minister’s “Coalition of the Willing” is approved.

They warn that sustaining an enduring operation in eastern Europe may require reservists for a minimum of two years, on a full time reservist service basis. The assessment reflects a wider manpower crisis in the regular Army. Several regiments are heavily under strength with more troops leaving than joining, and senior commanders concede that no significant deployment could be mounted without reserve support.

During the campaigns in Iraq and Afghanistan more than 40,000 reservists were mobilised, filling vital gaps in field hospitals, logistics and frontline infantry. Thirty-one were killed and 42 injured. Subsequent reforms have seen reservists more firmly embedded within the fabric of the regular forces.

A senior source at Army Headquarters said: “Almost everywhere you look we are short of people, even special forces. Reservists are already serving in the Falklands, on United Nations duty in Cyprus and in Iraq.

“If the government decides we are going to join a Peace Support Operation in Ukraine we will need the reserves, just as we did in Iraq and Afghanistan. We simply cannot sustain operations without them.”

More than 2,300 are employed full time on administrative and training duties under so-called “full-time reserve” service contracts.

A Ministry of Defence document obtained by the Sunday Express shows that more than 900 regular posts in Britain remain unfilled and are being advertised to reservists, ranging from junior instructors to senior command appointments.

Mobilisation is handled through the Reserve Training and Mobilisation Centre in Nottingham, where volunteers or specialists are formally called up and issued with a regular Army contract for the period of their service.

Doctors and engineers can receive enhanced pay to reflect their civilian expertise, through mass mobilisation has in the past placed pressure on public services such as the NHS.

Recruitment to the reserves remains a challenge, however. Official figures show 4,280 departures in the past year.

But senior officers believe the prospect of a Ukraine deployment could revive interest, pointing to Iraq and Afghanistan when applications rose during large-scale operations. The proposals come as other European nations move to bolster their manpower.

Germany’s cabinet has approved plans for voluntary national service, widely seen as paving the way for a return to conscription. France has expanded its reserve intake and the Baltic states are considering longer compulsory terms.

Britain already has more than 1,200 regular troops deployed in the Baltic States on Operation Cabrit, alongside NATO allies, with others stationed in Cyprus, the Falklands, Kenya, Oman and Germany. Adding a Ukrainian reassurance force would further stretch manpower at a time when the Army is struggling to meet existing commitments.

Speaking recently to the Sunday Express, former head of Joint Forces Command General Sir Richard Barrons said the British Army could spare up to 7,000 troops for a Ukraine reassurance operation – though sources say the figure is likely to be around 4,500.

Gen Barrons, who co-authored the Government’s recent Strategic Defence Review, added: “They must include reservists because it’s good for the reserve to be exposed to a live operation.

“Some will be used for training Ukrainians, but an element will be in the east because we are going to need to have skin in the game.

“They would require a nine-month rotation because six months is too short because it’s going to be a lot of work.”

A Ministry of Defence spokesperson said: “We will not comment on speculation around force composition, as the only person that will benefit is Putin.

“The UK, alongside France, continues to lead work on the Coalition of the Willing, which will strengthen Ukraine’s path to peace and stability by securing their skies, supporting safer seas, and regenerating Ukraine’s own forces.”