TORY MPs will today demand that in a dangerous world David Cameron pledge to maintain UK defence spending at a minimum of two per cent of GDP in the years ahead.

Pressure from the US Government as well as military chiefs at home has been added to after Anders Fogh Rasmussen, the former Nato Secretary General, claimed the Prime Minister had assured him the "UK would stay above the two per cent" level. He warned it would be "almost a dangerous signal" to other Nato allies if Britain went below the alliance's benchmark.

Luke Coffey, a former Ministry of Defence special advisor, noted how Mr Cameron had previously promised spending would rise after 2015 following cuts in 2010. But neither the Tories nor Labour have thus far committed to maintaining the two per cent target in the next parliament.

Sir Peter Wall, the former Army chief, also said the "significant reduction" in the defence budget early in this parliament had been made "against the expectation and an undertaking that the budget would increase when the economy started to turn the corner and improve".

Amid suggestions that Ministers had been asked to look at whether expenditure on intelligence services could be counted as defence spending to keep the UK above or close to the two per cent level, No 10 stressed the UK Government would stick to Nato guidelines when deciding what counted towards meeting the alliance's target on defence expenditure.

On a trip to Washington to meet his US counterpart, Michael Fallon, the Defence Secretary, stressed how the UK was one of only four member states meeting the Nato two per cent of GDP target but refused to say whether his party would commit to this beyond 2016.

He told a think-tank in the US capital, "You want Europe to do more to pay its way in defence; so do we. You want to see an end to the decline in Europe's defence spending that has a quarter of the alliance spending less than one percent of GDP on defence and 20 of the 28 members spending less than 1.5 per cent; so do we."

Meanwhile, Mr Cameron has insisted Triden and its replacement are "non-negotiable."

During Commons question-time, Tory backbencher Julian Lewis, referring to the possibility of a Lab-SNP alliance following the General Election, said: "The Scottish National Party have been licking their lips in public at the prospect of blackmailing one of the two main parties into delaying or abandoning the replacement of the Trident submarine.

"Will the Prime Minister confirm that if he is still Prime Minister in 2016, as he should be, that he will certainly ensure that the main-gate contracts for four successor submarines are signed that year?"

Mr Cameron replied: "I can reassure him; for me, Trident and its replacement are non-negotiable. They are an absolutely vital part of this nation's security."

Elsewhere, Angus Robertson, the defence spokesman for the SNP, which is strongly opposed to the renewal of Trident, said in the post-election defence review "let's put everything on the table, including Trident, and let's get the priorities right".