David Cameron must honour his commitment to restore spending on defence amid concerns that the West is looking dangerously weak in the eyes of Vladimir Putin, a former armed forces chief warned yesterday.

Air Chief Marshal Lord Stirrup, a former chief of the defence staff, said the Russian President was “testing” the resolve of Britain and its Nato allies in the stand-off in Ukraine and with increasingly aggressive flights by Russian bombers off the UK, raising the risks of a mid-air collision.

He suggested that Britain should now consider supplying arms to the Ukraine government to counter the superior firepower of the separatist rebels who are being supplied by Moscow.

“Nato over the years, in the eyes of Putin at least, has become weak. Putin views the West as weak all round and that is a very, very dangerous position for us to be in,” he told Sky News’s Murnaghan programme.

Putin views the West as weak all around and that is a very, very dangerous position

“My concern is that Nato is not spending enough resource on defence. If Nato is weak or is perceived to be weak by Putin then the risk of miscalculation is much greater and our security is much lowered as a result.”

Lord Stirrup, who was head of the armed forces at the time of the 2010 strategic defence and security review, said the “swingeing cuts” to defence spending had left the forces “right on the limit” and needed to be reversed.

“The 2010 defence review was predicated on defence expenditure increasing in real terms every year after 2015.

“The Prime Minister acknowledged that when he announced the outcome of the review in the House of Commons,” he said.

“We now have to make sure that those undertakings are kept. It was important in 2010. It is much more important now.”

He said that recent flights by Russian long-range Bear bombers off Bournemouth and the coast of Cornwall were part of a strategy to probe UK defences.

“They are becoming more aggressive. These aircraft – Russian Bears for example – are not going on these very flights simply as joy rides. They are mission rehearsals,” he said.

“These aircraft launch stand-off missiles against western targets and just as they used to do in the Cold War they are practising those profiles.

“They are testing us, they are testing our defences, they are testing our reactions and they are engaging to a degree in a game of chicken, and that’s very dangerous.

“So, we are seeing the possibility of mid-air collision not between, I think, RAF and Russian aircraft but between Russian aircraft and civilian aircraft increasing.”

Lord Stirrup joined former defence secretary Liam Fox in suggesting that the West should now consider arming the government in Kiev.

“I think that is something we need to think about very carefully. If we were to do that it needs to be part of a long-term strategy, not just a short-term, knee-jerk response,” he said.

That was ruled out, however, by the Leader of the Commons, William Hague, who said that the right approach was to ratchet up sanctions if Russia continued to de-stabilise Ukraine.

“We are not planning as the UK to send arms to Ukraine. It hasn’t been our approach in any recent conflict in recent years to send arms into those conflicts,” Hague, who was foreign secretary until last year, told BBC One’s The Andrew Marr Show.

“We want to see a diplomatic solution. We believe that long-term economic price to Russia and all of those other penalties are the right penalties for Russia to pay.” The chairman of the Commons Defence Committee, Conservative MP Rory Stewart, said that all the political parties now needed to commit to maintaining the Nato target of spending 2 per cent of GDP on defence.

“This is absolutely central. Every party must in their manifesto commit to 2 per cent spending on defence,” he said.

“That is what Nato is about. It is passing the message to Putin that Nato is a serious organisation that is prepared to defend its territory.

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