Marines destroyed 20 truck loads of weaponry
A government spokesman said British troops in Afghanistan were "100% clear" the weapons had been al-Qaeda's.
He denied they had been those of Ibrahim Omari, an Afghan ally, who claims to have been storing the weapons for 15 years.
The weapons were blown up at the weekend as part of Operation Snipe and said to belong to al-Qaeda forces in the Paktia province.
A Downing Street spokesman said the weapons had passed through various hands but belonged to al-Qaeda forces at the time of destruction.
Operation Snipe was hailed as a success after four caves stacked with ammunition - 20 truck loads of weaponry - were discovered.
The weapons' destruction was described as the biggest explosion of its kind since World War II and a major blow to enemy forces.
But Mr Omari told the Daily Telegraph he informed forces of the cache four months ago and last week led British officers to the site.
Enemy forces
The former anti-Soviet fighter is the head of tribal affairs in the Gardez city, one of the coalition forces' forward bases.
He said: "The weaponry was nothing to do with al-Qaeda - my own people had been guarding it for years.
"I had been hoping to save it for when we have a national army established."
He added most of the weapons had been supplied by the United States when it backed Afghanistan's anti-Soviet resistance in the 1980s.
"Now the Americans' friends, the British, are blowing it up."
But a Ministry of Defence spokesman said: "There was a great deal of intelligence suggesting this was a site for ammunitions and equipment in al-Qaeda hands.
"The last known owners of the weapons are al-Qaeda and the dramatic and final end to the weaponry means it was an immensely successful operation.
"To take these weapons out of the hands of opposition forces, and future enemies, is extremely important."