The lawyer for US vice-president Dick Cheney claimed today that the Congress lacks any authority to examine his behaviour on the job.

The exception claimed by Cheney's counsel came in response to requests from congressional Democrats that David Addington, the vice-president's chief of staff, testify about his involvement in the approval of interrogation tactics used at Guantanamo Bay.

Ruling out voluntary cooperation by Addington, Cheney lawyer Kathryn Wheelbarger said Cheney's conduct is "not within the [congressional] committee's power of inquiry".

"Congress lacks the constitutional power to regulate by law what a vice-president communicates in the performance of the vice president's official duties, or what a vice president recommends that a president communicate," Wheelbarger wrote to senior aides on Capitol Hill.

The exception claimed by Cheney's office recalls his attempt last year to evade rules for classified documents by deeming the vice-president's office a hybrid branch of government - both executive and legislative.

The Democratic congressman who is investigating the legal framework for the violent interrogation of terrorist suspects, John Conyers, has asked Addington and several other top Bush administration lawyers to testify. Thus far all have claimed their deliberations are privileged.

However, Philippe Sands QC, law professor at University College, London, has agreed to appear in Washington and discuss the revelations in Torture Team, his new book on the consequences of the brutal tactics used at Guantanamo.

Excerpts from Torture Team were previewed exclusively by the Guardian earlier this month.

Two witnesses sought by Conyers, former US attorney general John Ashcroft and former US justice department lawyer John Yoo, claimed that their involvement in civil lawsuits related to harsh interrogations allows them to avoid appearing before Congress.

In letters to attorneys representing Ashcroft and Yoo, Conyers shot down their arguments and indicated he would pursue subpoenas if their clients did not testify at his May 6 hearing.

"I am aware of no basis for the remarkable claim that pending civil litigation somehow immunises an individual from testifying before Congress," Conyers wrote.

Conyers, who chairs the House of Representatives judiciary committee, also questioned the reasoning of Cheney's lawyer in a letter to Addington.

"It is hard to know what aspect of the invitation [to you] has given rise to concern that the committee might seek to regulate the vice president's recommendations to the president," Conyers wrote.

"Especially since far more obvious potential subjects of legislation are plentiful," he added, mentioning several: US laws on the use of torture on terrorist suspects, the 15-year-old War Crimes Act, and the rules that allowed the Bush White House to receive legal advice from a specialised office within the justice department.

