The head of an influential parliamentary arms committee today said he was "very concerned" at the government's refusal to cooperate with the US criminal investigation into allegations of corruption against BAE Systems.

More than two months after the US justice department formally requested assistance in its investigation of Britain's biggest arms company, the home secretary, Jacqui Smith, has failed to pass on the request to the Serious Fraud Office.

The lack of action has led US investigators to believe that the British are obstructing the inquiry.

The Labour MP Roger Berry, chairman of the arms export select committee, said he found the government stance "difficult to understand".

"I am obviously very concerned about the report and the allegation," he told Guardian Unlimited.

"If it's true that the government is not responding positively to a request from a US investigation, it's an extremely unusual way to behave towards a major ally which has at least as much interest in the future of Saudi Arabia as the UK."

US investigators want access to files gathered by the SFO during its major inquiry into £1bn of payments to Prince Bandar of Saudi Arabia and into other bank accounts linked to the Saudi royal family.

Ms Smith's go-ahead is required before the SFO can cooperate.

The SFO was forced to drop its criminal investigation earlier this year by the then prime minister, Tony Blair, who said it would have damaged the UK relationship with the Saudi regime, and consequently posed a threat to British security interests.

However, Mr Berry said: "I was not convinced by that argument at the time, but it's even more difficult to be persuaded by the argument now, given that our major ally is trying to facilitate an investigation into alleged criminal activity."

He said it was imperative that the rule of law was upheld and that it was "applied equally".

"It's extremely difficult to understand," he said. "I hope the government will squash the rumour or for goodness sake explain what the reasoning is."

The Campaign Against the Arms Trade (CAAT) and the environmental organisation Corner House will go to the high court on November 9 in a bid to get the SFO to reopen its inquiry into BAE.

The CAAT spokesman Symon Hill said: "It's imperative that the UK hands over relevant information to the US inquiry but it's still imperative that the UK inquiry is reopened. This is a perfect opportunity for Gordon Brown to demonstrate his independence from Blair and that he won't allow the UK to be bullied by arms dealers."

The Home Office said its failure to pass on the request to the SFO was "not unprecedented" but could not give any example of similar behaviour.