Britih Foreign Office diplomats withdrew from the Libyan city in June following an assassination attempt on the British ambassador but reached an agreement with the US to leave weapons and vehicles at the American compound.
The lightly-guarded US site was overrun by Islamist militants in an attack that killed Ambassador Chris Stevens and three other Americans last month.
The Foreign Office has confirmed to the UK daily newspaper The Daily Telegraph that the equipment is still unaccounted for a month after the Benghazi assault.
Dozens of attackers tore through the American compound during the September 11 attack and since then Libyans have often roamed freely through its remains, raising the possibility that the British weapons have already been looted.
Details of the agreement between the British and American diplomatic security teams emerged during a Congressional hearing into the Benghazi attack on Wednesday.
Lieutenant Colonel Andrew Wood, the leader of a US special forces team in Libya, said a deal had been made where the British would "leave their weapons and vehicles on our compound in Benghazi".
"They would come back and at times withdraw their weapons and vehicles and then return them and leave," he said.
Britain's decision to remove diplomats from the eastern Libyan city was repeatedly raised during the hearing, as Republicans demanded to know why the US had stayed even when its close ally had decided the situation was no longer safe.
Lt Col Wood said that the withdrawal of British diplomats and Red Cross workers should have been a warning to US diplomats. "I almost expected the attack to come. We were the last flag flying, it was a matter of time," he said.
Sir Dominic Dominic Asquith, Britain's envoy to Libya, survived a rocket attack on his on his convoy as it drove through Benghazi on June 11 but two of his bodyguards were injured. The ambush prompted the Foreign Office to close its office and remove British staff.
A Foreign Office spokeswoman would not give details of the British equipment in Benghazi. "Following the suspension of our office in Benghazi the US consulate agreed to store some equipment on our behalf. We are working with the US to establish what, if anything, has happened to this equipment," she said.
The US State Department did not return a request by the newspaper for comment.
Meanwhile, the mother of one of the Americans killed in Benghazi said she was "repeatedly lied to" by the Obama administration and given no details about her son's death.
Pat Smith, whose son Sean died alongside Mr Stevens, told CNN: "'I look at TV and I see bloody hand prints on walls, thinking: 'My God, is that my son's? I don't know if he was shot. I don't know - I don't know."