Hague said a shipment of arms intercepted in a recent operation by Afghan and international forces in Nimroz in southern Afghanistan were supplied by Iran and destined for the Taliban insurgency.
It is believed that British special forces were involved in the seizure of the arms convoy on February 5.
Hague said: “The detailed technical analysis, together with the circumstances of the seizure, leave us in no doubt that the weaponry recovered came from Iran. This is completely unacceptable.
“It is at odds with Iran’s claim to the international community and to its own people that it supports stability and security in Afghanistan.”
He said the weaponry was “clearly intended to provide the Taliban with the capability to kill Afghan and ISAF soldiers from significant range.”
Hague gave no further details of the weaponry, but officials in Afghanistan said International Security Assistance Force troops had seized 48 122mm rockets, which have a range of 20 kilometres (13 miles), in Nimroz, which borders Iran.
They are being linked to Iran’s elite Revolutionary Guards.
Iran rejects it was behind the shipment, with a foreign ministry spokesman describing the allegations last week as “baseless and unacceptable.”
Hague said Britain’s ambassador in Tehran has raised the issue with the Iranian foreign ministry.
A total of 358 British troops have been killed since operations in Afghanistan began in October 2001. At least 314 died in hostile action, often by bombs laid by the Taliban.
