Prime Minister David Cameron proclaimed yesterday that Britain's military is a "first-class player" after criticisms from a former US defence secretary - but immediately faced tough questions over his priorities from campaigners.

Robert Gates said an erosion of Britain's defence forces had reduced its ability to be a "full partner" to the United States across the whole range of military operations.

But Mr Cameron insisted the Ministry of Defence budget remained the fourth largest globally and promised more investment was being made to enhance its future capabilities.

During a visit to the Crossrail project in London, Mr Cameron said: "I don't agree with him. I think he has got it wrong.

"We have the fourth largest defence budget anywhere in the world. We are investing in future capabilities.

"We are a first-class player in terms of defence and as long as I am Prime Minster that is the way it will stay."

Campaign group Stop the War said Mr Gates's and Mr Cameron's comments were symptomatic of the reality of the so-called "special relationship" - where Britain is expected to support US endeavours with troops and equipment.

Stop the War convener Lindsey German said: "I think that most people in this country don't like the 'special relationship.'

"Why are we spending more on the military than we are on education?

"Why, when we're not even the biggest economy in Europe, do we have the fourth largest defence budget?"

Mr Gates, who served under presidents Barack Obama and George Bush, said he lamented cuts in defence spending on both sides of the Atlantic.

He told the BBC Radio 4 Today programme: "With the fairly substantial reductions in defence spending in Great Britain, what we are finding is they won't have full-spectrum capabilities and the ability to be a full partner as they have been in the past.

"Because I lament our own defence cuts, I would say I also lament that reality in terms of Great Britain."

Chief of the Defence Staff General Sir Nicholas Houghton had warned last month that manpower was increasingly seen as an "overhead," and that Britain was in danger of being left with hollowed-out armed forces, with "exquisite" equipment but no soldiers, sailors and airmen to operate it.