Iran is conducting reconnaissance missions on Britain's nuclear power stations in preparation for retaliatory attacks should the West strike against Tehran's nuclear installations, a European security analyst told MPs yesterday.

The claim from Claude Moniquet, president of the European Strategic Intelligence and Security Centre, a Brussels think tank, came after David Cameron urged stronger international sanctions on Iran to force it to give up its nuclear ambitions. The call from the Conservative leader came amid reports Tehran was forging ties with al Qaeda in Iraq to prepare for a summer fight against coalition forces, and that it was arming Taliban insurgents fighting British troops in Afghanistan.

At a Westminster meeting, Mr Moniquet said his organisation had evidence Tehran had increased its intelligence agents across Europe. "We have serious signals that something is under preparation in Europe. Iranian intelligence is working extremely hard to prepare its people and to prepare actions," he said.

The security analyst said his think tank, which deals with European intelligence agencies, believed Iranian spies had carried out "reconnaissance of targets in European cities, including nuclear power stations". He added that Tehran appeared to be preparing to target "British citizens on the streets of London just as they kill British soldiers in the south of Iraq".

One of the MPs at the meeting, Patrick Mercer, the former Shadow Homeland Security minister, told The Herald that he believed the UK was in effect in an "undeclared war" with Iran given its activities in Iraq. Asked if he believed Mr Moniquet's assertion that Iranian agents were monitoring British nuclear power plants with a possible view to attacking them, Mr Mercer said: "Yes, I do because it is the second time I have heard this from disparate sources."

Tony Blair has already openly accused Iranian elements of backing insurgents in Iraq who target British forces. Last year, parliament's Intelligence and Security Committee warned of an "increased threat to UK interests from Iranian state-sponsored terrorism".

Yesterday, a government security official said Iran was active in espionage and likely interested in compiling information on European military and industrial targets, but he could not verify claims Tehran had conducted reconnaissance or increased its spies.