David Cameron must "repair the damage" to the UK's security by guaranteeing a minimum level of defence spending , the former head of the RAF has said.
Sir Michael Graydon said any prime minister who wants to be "remembered as a statesman" should take the opportunity to commit to maintaining the Nato target of spending a minimum of 2% of national income on defence.
Neither the Conservatives nor Labour pledged in their manifestos to meet the 2% target beyond 2015/16 after the next defence spending review.
Tory former security minister Baroness Neville-Jones said during the election campaign she was "willing to bet" a Conservative government would stick to the commitment, with the Prime Minister coming under r enewed pressure following his general election victory.
In a foreword to the latest UK National Defence Association ( UKNDA) report, retired air chief marshal Sir Michael wrote: "T he Prime Minister has now an opportunity to do what he and the other major party leaders felt unable to do in this election campaign, namely to repair the damage done to our defence and security in recent years and to our reputation as a serious contributor to world security.
"He can start immediately, by endorsing the House of Commons Defence Committee Report of March 17 this year - "Re-thinking defence to meet new threats", and in a simple statement make good his commitment to the Nato minimum of 2% of GDP.
"It is an opportunity which any prime minister who aspires to be remembered as a statesman should take."
The UKNDA, a group of former military chiefs and politicians, wants the Government to commit to the Nato target of 2% as a minimum for the rest of the parliament and protect the budget from public spending cuts.
Military chiefs should also be free to give their views publicly in line with the approach adopted in the United States so that MPs and the public "can be reassured of the adequacy of defence provision", the report adds.
A Government spokesman said: "With the second largest defence budget in Nato and the largest in the EU, the Government is committed to spending 2% of GDP on defence this financial year.
"Decisions on spending after the financial year 2015/16 will be determined in the next spending review.
"This Government was elected with a mandate to maintain the size of regular armed forces, to increase the equipment budget in real terms every year, and to renew our four nuclear ballistic submarines. These commitments will secure the shape and power of our armed forces and keep Britain safe."