The bombs, triggered when an infra-red beam is touched, have created havoc among British forces in southern Iraq. They release a projectile capable of penetrating armoured vehicles, against which the British army has virtually no defence.

The British official said that Iranian interference in Iraq could be related to British pressure on Iran over its suspected nuclear weapons ambitions. "It would be entirely natural that they would want to send a message 'Don't mess with us'," he said. An Iranian government spokesman rejected the British accusations and said it was opposed to the insurgency in Iraq.

The confrontation marks a hardening of relations between London and Tehran. Since 1997 the Foreign Office has been energetically engaged in trying to improve ties. But relations took a turn for the worse in June with the election as president of the Iranian hardliner, Mahmoud Ahmadinejad, and the rejection in September of a nuclear deal offered by Britain and supported by France and Germany. Iran threatened reprisals after the United Nations nuclear watchdog voted to refer Tehran to the UN security council for possible punitive sanctions. At last month's UN summit, Mr Ahmadinejad dismayed George Bush and Tony Blair with what they saw as a confrontational speech that dashed hopes of a nuclear deal.

In July three soldiers from the Staffordshire Regiment were killed by one of the devices while patrolling near the Iranian border. Five other British soldiers were killed by similar bombs this year, as well as two British security guards who were part of the diplomatic protection team.

The British official said the bombs were designed and manufactured by the Tehran-backed guerrilla group Hizbullah, based in Lebanon, and were channelled to Iraq via Iran. "Iran's motives certainly don't seem that benign. If Iran wants to tie down the coalition in Iraq, then that is consistent with supplying insurgent groups."

He said Iran was providing help not only to their co-religionist Shia insurgents but to Sunni insurgents too. "There is some evidence that Iranians are in contact with Sunni groups."

He specifically blamed the smuggling of the bombs to Iraq on the Iranian Revolutionary Guard Corps, a military organisation which has traditionally directed Iran's links with insurgent groups in the Arab world and which is answerable to Iran's highest executive body, the national security council. It is chaired by Mr Ahmadinejad, a former commander of the IRGC who replaced the moderate, pro-western former president, Mohammad Khatami.

The Iranian spokesman flatly rejected the accusations: "The stability of Iraq is of paramount importance to Iran. The Islamic Republic of Iran has always taken a position against insurgency and violence in Iraq," he said. "These claims have come after Iran raised her concern about possible British forces' support and links with some terrorist elements who crossed the Iranian border and were behind some explosions in the southern part of Iran." Similar Iranian assertions of British interference in Iran have been dismissed by Britain out of hand in the past.

The British official said there was little prospect of the insurgency, which is fiercest in the US-run Sunni Arab areas of Iraq, "dying away any time soon" and predicted a "spike" in violence in the run-up to an Iraqi referendum on the constitution on October 15 and parliamentary elections in December.

Britain has formally complained to Tehran over the supply of the bombs. The accusation yesterday is its most explicit charge yet of Iranian interference. In August, the British government highlighted the interception of a cache of conventional weapons being smuggled across the Iran-Iraq border. "We continue to press Iran on that and we continue to encourage the Iraqi government to do that," the official said.

Britain hopes to begin withdrawing some of its 8,000 troops next year, if security conditions permit.

There are differing views within the British intelligence community as to the level of Tehran's involvement. British military sources insisted last night there was no hard evidence that the explosives technology came from Iran. Defence sources suggested that blaming the IRGC for supplying the explosives technology was going too far. Other military officials said there was "so much expertise in Iraq" the bombs could have been made by former members of Saddam Hussein's security forces.

The difference in opinion may reflect concern on the part of the military that a sharpening confrontation with Iran could increase the chances of further attacks on British troops.