The former US vice-president Dick Cheney ordered the CIA to conceal a highly secret counter-terrorist programme from Congress for eight years, possibly in breach of longstanding oversight laws.

Democratic leaders in Congress are planning hearings to establish how and why information about the programme was withheld. The details have been revealed to members of intelligence committees but not been made public.

The revelation in the US press on Sunday that Cheney played a primary role in keeping the programme secret suggests that it would have been highly contentious. Attention has focused on reports earlier this year that he oversaw an assassination programme.

One member of an intelligence committee who was briefed on the secret operation last week said that Congress would have been unlikely to have approved it.

According to US intelligence officials quoted in the US media the CIA director, Leon Panetta, told congressional intelligence committees that information about the programme was withheld on Cheney's orders. Panetta told the committees that as soon as he learned of the programme's existence last month he shut it down.

The law requires the president to keep Congress "fully and currently informed of the intelligence activities", although it does allow information to be withheld about "exceptionally sensitive matters". However, it has been the accepted practice that the existence of even the most secret category of covert programmes is revealed to the "gang of eight" Democratic and Republican leaders of the two houses of Congress and their intelligence committees. That was not done on this occasion, apparently on Cheney's orders.

The nature of the programme has not been made public, although it does not involve the CIA's controversial use of waterboarding and other forms of torture. Nor is it about domestic intelligence.

In March, the respected investigative reporter Seymour Hersh revealed that he had uncovered evidence during research for an as-yet unpublished book that Cheney oversaw an "executive assassination ring" for years.

"It is a special wing of our special operations community that is set up independently. They do not report to anybody, except in the Bush-Cheney days, they reported directly to the Cheney office. ... Congress has no oversight of it," he said at the time.

"It's an executive assassination ring essentially, and it's been going on and on and on. Under President Bush's authority, they've been going into countries, not talking to the ambassador or the CIA station chief, and finding people on a list and executing them and leaving. That's been going on, in the name of all of us."

Whether or not the secret programme involved assassination, an insight in to how radical it might have been was offered by Peter Hoekstra, a leading Republican on the House of Representatives intelligence committee. He told the New York Times that he believed Congress would have approved it in the days immediately after the 9/11 attacks but would have backed away after that.

Dianne Feinstein, chair of the Senate intelligence committee, said today that there was no justification for withholding information about the secret operation and suggested that laws were broken.

"This is a big problem because the law is very clear. I understand the need of the day which was when America was in shock, when we had been hit in a way we never contemplated, where we had massive loss of life, where there was a major effort to be able to respond. But I think you weaken the case when you go outside of the law... That's something that should never ever happen again," she said.

The chair of the House intelligence committee, Silvestre Reyes, said its members were "affirmatively lied to" about the operation.

Republican members of Congress have suggested that the programme never got beyond the planning stages and therefore there was no need to inform the intelligence committees.

However, there is scepticism that the operation could have been in preparation for eight years and yet never put into effect.

In recent weeks Cheney has launched a vigorous public defence of his actions over revelations about the extent of waterboarding and other torture against suspected terrorists. Last week, a report by a group of inspector generals underscored Cheney's central role in "unprecedented" extensive covert wiretapping of Americans. The report questioned the programme's legal rationale and the excessive secrecy around it.

The CIA, meanwhile, could face further scrutiny after it emerged that the attorney general, Eric Holder, is considering appointing a special prosecutor to investigate allegations that the agency illegally tortured terrorism suspects.