The anonymous British official also claimed that Iran’s army was in contact with Sunni rebel groups fighting foreign forces in Iraq.
The comments came as Britain’s embassy in Tehran denied claims in the Iranian press that it was stirring up unrest in the southwest Iran, just across the border from where British soldiers are based in Iraq.
Iran’s foreign ministry spokesman Hamid Reza Assefi dismissed what he called the British “lies”.
“If they have proof, they only have to provide it,” local media quoted him as saying. “But they have no proof. They are the ones to blame for the instability in Iraq, and they accuse others.”
The spokesman said Iraqi officials had acknowledged the “positive and constructive policies of Iran in Iraq”.
The British official said the Iranian action could be an attempt to warn off London over its demands that Tehran abandon its controversial nuclear program.
“It would be entirely natural that they would want to send a message: ‘Don’t mess with us’,” the anonymous official said.
Britain believed the Revolutionary Guards had been responsible for supplying explosives technology used in a series of deadly attacks on British troops earlier this year that killed eight British soldiers.
“We think it has come from Lebanese Hezbollah via Iran,” the official said, refusing to say whether Britain believed the actions were on the orders of the government in Tehran.
In August, a US intelligence official told news agency AFP anonymously that Washington believed a cache of bombs seized in Iraq had been smuggled into the country by the Revolutionary Guard.
Although Iran is Shia Muslim, the official said it appeared elements from the country were in contact with Sunni insurgent groups.
“There is some evidence that the Iranians are in contact with Sunni groups. I don’t think it is for a benign purpose,” the official said.
“If part of the aim was to tie down the coalition in Iraq, it would be entirely consistent with supporting those groups.”
London had protested to Tehran, which had denied responsibility, the official added.
Britain has around 8,000 troops in Iraq, mainly based in the south of the country. Since the March 2003 invasion of Iraq, 95 British soldiers there have died.
Last year Iran held the crews of three Royal Marine river patrol boats from Iraq for three days, charging the vessels had strayed into Iranian waters.