British special forces in Afghanistan have intercepted an Iranian shipment of rockets to the Taliban that would have allowed them to double the range of their attacks, western diplomats have said.

The rockets were discovered after an intelligence tip-off on 5 February when British special forces and Afghan troops stopped a convoy in Nimruz province, in the south-west of Afghanistan bordering Iran and Pakistan, the officials said. A shoot-out involving the special forces left several Taliban fighters dead.

The vehicles were found to be carrying 48 122mm rockets, which western sources described as "substantial weapons" with a range of more than 12 miles. A diplomat with knowledge of the arms shipment said that was double the range of the usual Taliban weapons.

Britain's foreign secretary, William Hague, issued a statement denouncing what he called "completely unacceptable" Iranian behaviour.

"I am extremely concerned by the latest evidence that Iran continues to supply the Taliban with weaponry – weapons clearly intended to provide the Taliban with the capability to kill Afghan and Isaf soldiers from significant range. The detailed technical analysis, together with the circumstances of the seizure, leave us in no doubt that the weaponry recovered came from Iran," Hague said. "It is not the behaviour of a responsible neighbour."

Liam Fox, the defence secretary, said in a statement: "This confirms my often repeated view of the dangers that Iran poses not only through its nuclear programme but it's continuing policy of destabilising its neighbours. Supplying weapons to help the Taliban kill Isaf soldiers is a clear example of the threat they pose."

Mark Sedwill, Nato senior civilian representative to Afghanistan, said: "These rockets represent a step-change in the lethal impact of weaponry infiltrating Afghanistan from Iran.

"All Afghanistan's neighbours have an obligation to prevent weapons from reaching the insurgents who will use them to kill Afghan civilians as well as Afghan and international forces."

The weapons were recently manufactured, rigorous technical and intelligence examination had shown, the sources said. "They were doctored and tampered with to look as they came from a third party but were of proven Iranian origin," an official said.

The rockets resembled Soviet-era weaponry, caches of which are still occasionally found in Afghanistan. An examination involving British specialists confirmed they were from Iran.

Markings had been removed on most but not all of the rockets. They had a green fuse plug, supposedly unique to Iranian-made rockets.

"There are at least elements of the Iranian regime who are determined to make life difficult for us," a European diplomat said. "We have always suspected that there was a flow of arms, but we can never be sure how much of it we are find and how much we are missing."

A thousand rounds of ammunition were also found in the convoy.

It is the largest confirmed shipment of Iranian arms to the Taliban since 2007, when armour-piercing bombs were found in a vehicle in the western province of Farah.

Tehran officially supports Hamid Karzai's government in Kabul but Iran's Revolutionary Guard has long been accused of smuggling arms to the Taliban to use against the US-led force in Afghanistan, Isaf.

The disclosure of the operation, which took place more than a month ago, comes less than a week after the SAS and British intelligence were seriously embarrassed by the disclosure that UK special forces had been captured by rebels in eastern Libya.

British officials suggested it took some time to conduct detailed tests on the rockets.

Associated Press quoted an intelligence official as saying that a high-level Afghan Taliban leader had travelled to Iran in the past two weeks to ask a leader of the Quds force of Iran's Revolutionary Guard for more powerful weapons to attack Afghan and Nato troops in the spring and summer.

The official said the logistics of the shipment were arranged by a Taliban facilitator based in Iran.