DAVID Cameron has hit back at mounting pressure over spending on UK defence, insisting that Britain is "not shrinking its role in the world" as a result of cutbacks.

Ahead of a key Commons debate today when a backbench attempt will be made to enshrine in law defence spending of two per cent of GDP - which the UK is currently meeting - the Prime Minister has come under pressure on three fronts.

General Sir Peter Wall, the former head of the British Army, warned that the funding squeeze was "playing out" in the UK's approach to dealing with Vladimir Putin, leaving the UK reticent to tackle Russian incursions into its airspace and offshore waters.

Samantha Power, the US ambassador to the United Nations, used a speech in Brussels this week to voice concern over declining defence spending in European countries at a time when an increasing number of missions required them to supply troops.

Increasingly vociferous Tory MPs want the UK Government to commit to meeting Nato's target for member-states to spend two per cent of national income on defence by enshrining it in law like the commitment to spend 0.7 per cent of GDP on overseas aid.

A report by the Royal United Services Institute said it was inevitable that Britain's defence spending would drop below the Nato target in the face of continuing austerity cuts. It predicted up to 30,000 service personnel could go - with the Army likely to bear the heaviest cuts - leaving the armed forces with a combined strength of just 115,000 by the end of the decade.

But Mr Cameron was defiant, saying: "Anyone who feels that Britain is somehow shrinking its role in the world; that's not the case."

He stressed that Britain had met the two per cent target throughout this five-year Parliament and was due to do so in the coming years.

"What we have done with the defence budget is we froze it in cash terms at around £36bn; that's the fifth biggest defence budget in the world and the second biggest in Nato.

"We have made very specific pledges to increase in real terms the equipment budget, which is absolutely vital; that's the aircraft carriers and the Type 26 frigates and the destroyers and hunter-killer submarines and the rest."

Asked about prominent figures raising concerns about defence spending, Mr Cameron suggested sometimes they had a book to talk about and other times they were just expressing their views.

He added: "I have responsibility to make sure we make the right decisions about defence and other security spending. I look at these things in the round, so I am also concerned about the budget for MI5, the Secret Intelligence Service, GCHQ, counter-terrorism policing. To me all of these things are part of our national defence."