Militants in Iraq and Syria pose the gravest threat to Britain today, David Cameron has warned.

The Prime Minister said the police service and spy agencies have been ordered to focus their efforts on the Islamic insurgents, particularly those returning to Britain.

It comes after MailOnline revealed more British citizens have signed up to fight in Iraq and Syria than joined the Army Reserve last year.

Ministers have been caught off-guard by the speed of the insurgency by the Islamic State of Iraq and the Levant (Isis) sweeping through Iraq, with more evidence emerging of brutal executions.

Several hundred Britons have travelled to the region to join militants in fighting, but just 170 extra reservists enlisted over the past year.

Today Mr Cameron insisted the prospect of battle-hardened jihadists returning to the UK was a ‘real threat to our country’.

Speaking at a joint press conference with Chinese premier Li Keqiang, Mr Cameron said: ‘No-one should be in any doubt that what we see in Syria and now in Iraq in terms of Isis is the most serious threat to Britain's security that there is today.

‘The number of foreign fighters in that area, the number of foreign fighters including those from the UK who could try to return to the UK this is a real threat to our country.

‘We will do absolutely everything we can to keep our people safe.

'That means stopping people from going, it means arresting people who are involved in plots, it means focusing our security, our policing, our intelligence effort on to that area of the world, on to those people.’

Britain is now looking to Iran for help in tackling the crisis in neighbouring Iraq.

Foreign Secretary William Hague announced Britain is to reopen its embassy in Tehran for the first time since it was ransacked by an angry mob in 2011.

'There has never been any doubt in my mind that we should have an embassy in Tehran if the circumstances allowed,' Mr Hague told MPs.

'Iran is an important country in a volatile region, and maintaining embassies around the world, even under difficult conditions, is a central pillar of the UK's global diplomatic approach.'

He stressed there were 'a range of practical issues' that had to be resolved before the base could start to operate again.

'Inevitably, the initial embassy presence will only able to offer a limited range of services at first. For the time being, Iranians will still need to apply in Abu Dhabi or Istanbul for visas for travel to the UK,' he added.

Young jihadi: A child is pictured on an ISIS-supporting Twitter feed wearing a balaclava and holding a sign saying 'God is greatest'

A senior envoy was appointed as Britain's non-resident charge d'affaires last year, as relations improved after perceived moderate Hassan Rouhani was elected president. There has also been substantial progress reducing tensions over Iran's nuclear ambitions.

However Mr Cameron rejected the idea that the diplomatic rapprochement was connected to the crisis.

'Britain believes in and I believe in step by step building our relationship with Iran because we need to have proper dialogue with that country.

'We are having dialogue over the nuclear weapons issue and I think we should be having dialogue with it on issues of regional security,' the premier said.

'Obviously our relationship was at a low point after the appalling things that happened with respect to our embassy, but it is right step by step with a clear eye with a hard head, to rebuild that relationship.

'Now we would be doing that anyway irrespective of what is happening in Iraq, but I think what is happening in Iraq is certainly not a reason for not taking that step.'

Charlie Cooper, an expert at the anti-extremist think tank, Quilliam Foundation, said the number of Brits signing up to fight in the Middle East would only grow.

He said: 'People are going from Britain and the EU to fight for ISIS - the estimate is more than 500 and that number will probably rise.