Britain’s Foreign Secretary William Hague leaves following a Cabinet meeting at Downing Street in London yesterday. Photo: Reuters/Kieran Doberty

Insurgencies led by religious extremists in Iraq and Syria are the most serious danger Britain faces today, David Cameron has warned.

The Prime Minister insisted the prospect of battle-hardened jihadists returning to the UK was a “real threat to our country”.

But he denied that the Government was restoring diplomatic links with neighbouring Iran in a bid to win Tehran’s help to quell the spread of Islamic State of Iraq and the Levant (Isis) militants.

Speaking at a joint press conference with Chinese Premier Li Keqiang, 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 serous 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.”

The comments came after Foreign Secretary William Hague said the “circumstances are right” to reopen the embassy in Tehran, which shut after being ransacked by a mob protesting against sanctions 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

“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.

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.

The move comes as the insurgency by the Islamic State of Iraq and the Levant (Isis) rages on, with more evidence emerging of brutal executions. Experts suggest Iran could play a vital role in helping to shore up the government in Baghdad.

But 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.