The equipment was found by Israeli troops in Hezbollah command bunkers in southern Lebanon. Each set was stamped “made in Britain”.

The Israelis made representations to the Foreign Office after it was revealed that Britain had sold 250 night-vision systems to Iran in 2003 for use against drug smugglers.

Foreign Office officials said early indications seemed to suggest that the night-vision equipment found by the Israelis was not part of the batch sold in 2003 to Iran. However, thorough checks were being made to compare serial numbers on the equipment found in the Hezbollah bunkers with those on the ones exported legitimately to Iran.

The Iranians are the prime sponsor of Hezbollah, and the Israeli authorities are demanding to know whether the equipment sold to Iran three years ago ended up in the hands of Hezbollah, which killed 117 Israeli soldiers during the month-long clashes in Lebanon.

A Department of Trade and Industry official said night-vision equipment of military specification required an export licence. The investigation will look into whether any British company might have breached export regulations.

The batch of 250 night-vision systems were given a special export licence in 2003 because they were intended to be used by Iranian police trying to stem the flow of heroin and opium from Afghanistan into Iran. Although there is what amounts to an arms embargo against Iran, aimed principally at stopping the export of equipment that could benefit Tehran’s suspected nuclear weapons programme, the request for night-vision equipment was approved in recognition of the counter-narcotics work.

When the export was agreed, Mike O’Brien, then Junior Minister at the Foreign Office, told the Commons: “The goods are for the use on the Iran-Afghanistan border against heroin smugglers.” He said there was “no risk of these goods being diverted for use by the Iranian military”.

If any of the equipment has been diverted to Hezbollah, it would be a serious embarrassment for the Government. Hezbollah’s “external security”, the military wing of the militant organisation, is proscribed as a terrorist group. The Government has also made clear its support for Israel’s struggle with Hezbollah and has approved the transit of bunker-busting bombs and missiles for the Israelis from the US through British airports.

Liam Fox, the Shadow Defence Secretary, said: “If this turns out to be true, and Iran supplied backing for Hezbollah, it will have consequences for any future military exports to Iran. And it points the finger all the more strongly at Iranian involvement in destabilising the Middle East.”

One set of the equipment was found by Israeli forces in the southern Lebanon village of Mis-a-Jebel on August 10, in a house belonging to a 60-year-old man whose four sons are all Hezbollah fighters.

One was described as a Thermo-vision 1000 LR system with a serial number 155010, part number 193960. Other equipment, including radios also thought to be British, and sophisticated recording and monitoring devices, were found.

Israeli commanders had complained that night-time operations in the border region had been hampered by the ability of Hezbollah fighters to observe and counter their moves. In more than six days of fighting around the village of Mis-a-Jebel, the Israelis lost six soldiers and 20 more were injured.

Lieutenant-Colonel Olivier Radowicz, an Israeli commander, said: “The night-vision unit was used to observe the movement of troops. You can also record what you are watching. Then it is connected to a computer. You can obtain a perfect intelligence picture in real time. It is then connected to firing systems.”

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