The Foreign Office has spent a lot of energy over the past few days trying to play down a story reported in The Sunday Times two weeks ago and repeated by a senior British official in remarkably candid terms on Wednesday.

The official cannot be named under the conditions of the "background briefing" he gave to reporters at the Foreign Office. But it can certainly be said that he was in a position to know a great deal about what was going on in Iraq.

He was asked whether it was true that Iran was supplying the Shia militias with roadside bombs triggered by infra-red tripwires, just like those used to protect valuable items in museums or banks, bombs that have killed six British soldiers in southern Iraq over the past few months.

The Foreign Office, which has carefully fostered its links to Tehran, was aghast when the official not only confirmed this was true but went on to accuse the Iranian Revolutionary Guards Corps of smuggling them across the border into Iraq.

The problem was that, as Tony Blair was forced to admit yesterday, British intelligence officials are as sure as they could be that this is going on but have no categorical evidence to prove it.

Suspicions were first alerted in mid-July when three British soldiers were killed by a new type of roadside bomb. It bore all the hallmarks of devices used previously by Hezbollah who obtained them from Iran.

As The Sunday Times reported two weeks ago, the SAS was sent to Basra to track the bombs as they crossed the border, leading to the arrest of two militiaman suspected of involvement, and the furore over last month’s rescue of two SAS soldiers held by the militia as bargaining chips.

The Foreign Office might have its concerns that The Sunday Times report was confirmed in such a forceful way but with six British soldiers and two UK contractors killed by the bombs, and the UK mission in the south seriously damaged, Mr Blair’s words of warning arguably came far too late.

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