Image copyright PA Image caption Former Army head Lord Bramall was among the signatories of the letter

The UK is safer in the EU and better able to meet a range of global threats, including so-called Islamic State and Russia, ex-defence chiefs have said.

Twelve of the country's senior military figures said the UK was stronger in the EU at a time of "grave" challenges.

Downing Street later expressed regret that it had included a 13th signatory - Gen Sir Michael Rose - on the letter to the Daily Telegraph by mistake.

At least two defence ministers are backing the leave campaign.

A referendum on whether the UK should stay in the EU will be held on 23 June.

The intervention comes after leaders of some of Britain's biggest companies said leaving the EU would put the UK's economy at risk.

'Greater power'

The letter, released by Downing Street and signed by officers from all three services, including former Chiefs of Defence Staff Lord Bramall and Lord Stirrup, said that while Nato was the most important alliance for UK security, the EU was an "increasingly important pillar of our security".

It said Europe was facing "grave security challenges" from instability in the Middle East, the rise of so-called Islamic State and "resurgent Russian nationalism and aggression".

The letter also said: "Britain will have to confront these challenges whether it is inside or outside the EU. But within the EU, we are stronger.

"Inside it, we can continue to collaborate closely with our European allies, just as we did when we helped to force the Iranians to the negotiating table through EU-wide sanctions, or made sure that [Russian President] Putin would pay a price for his aggression in Ukraine."

Image copyright PA Image caption The Conservative MPs who want out of the EU include armed forces minister Penny Mordaunt

The letter said the UK's "firm veto over EU foreign policy decisions" meant it would not be forced to join EU initiatives that were "against our strategic interest" and would also allow it to block the rest of the EU from going ahead in such circumstances.

"Britain's role in the EU strengthens the security we enjoy as part of Nato, adds to our capability and flexibility when it comes to defence co-operation and allows us to project greater power on the international stage," the former defence chiefs added.

The BBC's defence correspondent Jonathan Beale said the letter made "very similar arguments" to those already put forward by Prime Minister David Cameron as to why Britain should remain in the European Union.

He said the claim the UK would be safer in the EU would be "hotly disputed" and one not everyone in the military would necessarily agree with - defence ministers Penny Mordaunt and Julian Brazier back the leave campaign.

Later on Wednesday, the prime minister's spokesman admitted that Number 10 had mistakenly included as a signatory the former director of special forces, Gen Rose.

The spokesman said the general had got in touch from New Zealand to inform Number 10 of the error.

Signatories of the letter:

Field Marshal Lord Bramall, former Chief of Defence Staff

Field Marshal Lord Guthrie, former Chief of Defence Staff

Admiral of the Fleet Lord Boyce, former Chief of Defence Staff

Marshal of the RAF Lord Stirrup, former Chief of Defence Staff

Admiral Lord West, former First Sea Lord

Admiral Sir Mark Stanhope, former First Sea Lord

General Sir Mike Jackson, former Chief of the General Staff

General Lord Dannatt, former Chief of the General Staff

General Sir Rupert Smith, former Deputy Supreme Allied Commander Europe

General Sir Richard Shirreff, former Deputy Supreme Allied Commander Europe

Lieutenant General Sir John Kiszeley, former Director General of the Defence Academy