Britain can no longer deploy a division overseas as it did during the Iraq War because of a 'gutting' of its military capabilities, a damning US report has warned.

Defence chiefs will struggle to sustain much more than a brigade – around 6,500 troops – in a future war because of a significant slashing in the number of troops, it says.

The Center for Strategic and Budgetary Assessments (CSBA) said London will also face 'severe constraints' on its ability to engage in air warfare overseas.

Those constraints have 'already been felt' in the campaign against Islamic State, to which the UK has only been able to make a 'very modest contribution', it says.

Stretched: UK defence chiefs will struggle to sustain much more than a brigade – around 6,500 troops – in a future war because of a slashing in the number of troops, a Washington-based think tank says. Pictured: Scottish Desert Rats platoon soldiers in Basra, Iraq, in 2003

The report, from a Washington-based think-tank, will infuriate ministers who have repeatedly emphasised the significant role the UK has played in the defeat of IS.

American experts found America's closest and most powerful allies had all seen their military power 'erode substantially in the past two decades'.

The report, 'Dealing with allies in decline', says: 'America's most important NATO allies have been gutting their military capabilities over the past quarter century.

'Indeed, the decline of UK military capabilities offers a particularly stark national example of the overall European trend.'

The authors note that how, during the 1990s and early 2000s, the UK was by far the most important US partner in conflicts, such as the Persian Gulf War to the Iraq War.

'Modest': The UK's constraints have 'already been felt' in the campaign against Islamic State, to which it has only been able to make a 'very modest contribution', the report says. Pictured: Scottish Desert Rats platoon soldiers inspect a car in Basra, Iraq, 2003

British forces peaked at 46,000 during the invasion phase and then fell away year on year to 4,100 in May 2009 when the UK formally withdrew from Iraq.

There are currently just 78,407 regular soldiers in the Army, down from 102,000 in 2010.

A division is typically between 25,000 and 50,000 troops, comprising of several brigades.

But the report warns: 'Significant reductions in mechanized capabilities and Army end strength mean that for the next several years, the United Kingdom will probably be able to deploy and sustain no more than a brigade (around 6,500 troops) in overseas combat missions.'

It quotes another study which states it would now be 'impossible' to 'field and sustain division-sized mechanised units with multiple fighter-bomber squadrons for support' like the UK did in 2003,

Disagreement: Defence Secretary Sir Michael Fallon has said 'Britain is playing a leading role, through our airstrikes, intelligence, surveillance and reconnaissance, and our training'

The report, published this month, also criticises the strength of the Royal Navy, saying it is still unclear whether there will be enough jets to fly off Britain's new aircraft carriers.

On the RAF, it adds: 'Moreover, with 'much of the RAF's airborne intelligence, surveillance, and reconnaissance (ISR) capability…on track to being demobilized,' and with RAF combat forces shrinking to the size of roughly five US, Air Force squadrons, London will face severe constraints on its ability to engage in sustained air operations overseas.

'Those constraints—combined with declining political will to use force overseas— have already been felt in the ongoing counter-ISIS air campaign, to which the United Kingdom (like many NATO allies) has only been able to make a very modest contribution.'

The report will infuriate ministers who have repeatedly emphasised the significant role the UK has played in the defeat of Islamic State. Pictured: Displaced Iraqis flee their homes as Iraqi forces battle with Islamic State militants, in western Mosul, Iraq

This erosion in military power of America's allies in contrast to Russia's build-up of forces means the US will struggle to defend its 'overmatched' allies.

America's traditional responsibility as guarantor of stability and security in Europe and East Asia is becoming 'more difficult to uphold', the report warns.

It says: 'In the event conflict occurs, the United States will face even greater challenges in defending its increasingly overmatched allies in these regions.'

The Conservative manifesto pledges to 'maintain the overall size of the armed forces, including an army that is capable of fielding a war-fighting division'.

An MoD spokesman said: 'The Army, in line with the Strategic Defence and Security Review of 2015, is ready and capable of deploying a potent, large scale, war fighting force at divisional level with sufficient notice.'

Defence Secretary Sir Michael Fallon has said 'Britain is playing a leading role, through our airstrikes, intelligence, surveillance and reconnaissance, and our training'.