The Central Intelligence Agency has admitted "concealing significant actions" from Congress for years during the Bush administration, prompting the chairman of the House of Representatives intelligence committee to accuse the agency of having "affirmatively lied".

The admission raised fresh questions over what political pressure was applied to the CIA to manipulate and distort intelligence in order to mislead Congress and the public over a range of issues from Iraq's alleged weapons of mass destruction to the use of torture against al-Qaida detainees.

Members of Congress say the CIA director, Leon Panetta, made the admission at a closed session last month. At the time, a political feud had broken out between the agency and the speaker of the house, Nancy Pelosi, after she accused it of intentionally misleading her over the repeated waterboarding of the al-Qaida detainee, Abu Zubaydah.

The CIA's admission came to light after seven Democratic members of the House intelligence committee wrote to Panetta demanding he correct a statement in May in which he denied Pelosi's assertionsk, saying that it is not the agency's "policy or practice to mislead Congress".

"Recently you testified that you have determined that top CIA officials have concealed significant actions from all members of Congress, and misled members for a number of years from 2001 to this week," the letter said. "This is similar to other deceptions of which we are aware from other recent periods."

Silvestre Reyes, the chairman of the House intelligence committee, wrote a separate letter this week to the leading Republican on the panel. "This committee has been misled, has not been provided full and complete notifications, and (in at least one occasion) was affirmatively lied to," he wrote.

The politicians did not reveal the precise nature of the deceptions but said they were not trivial.

"It's serious stuff," Rush Holt, a New Jersey Democrat who chairs an intelligence subcommittee, told the Wall Street Journal. "Our reason for writing the letter in the first place has to do really with the integrity of Congress and the balance of powers."

However Republicans on the intelligence committee challenged the Democrats characterisation of the extent of Panetta's admission.

It is not clear if the latest revelations will improve the increasingly poisoned relationship between the Democratic-controlled Congress and the CIA, particularly over the agency's insistence that it told Pelosi about waterboarding when she says it did not.

Bradley Blakeman, a Republican strategist who worked on President Bush's White House staff, said that if the CIA has admitted to serious failings there should be a congressional investigation.

"If we are to believe that the CIA intentionally mislead Congress on matters of material intelligence activities, then, this is not the last we should hear of this. Congress should act with due speed to hold hearings, the attorney general should have the justice department open an investigation, and the inspector general of the CIA should do the same," he said.

But Blakeman said he suspects the statements are political, aimed at further blaming and embarrassing the Bush administration.

"If Pelosi thinks this vindicates her she is sorely mistaken. It may just re-kindle her own misleading statements with regard to what she knew and when she knew it with regard to interrogation techniques," he said.

The latest revelations come shortly before Congress is to begin debating a bill that will remove the president's right to decide when to give the full intelligence committees access to the most sensitive intelligence. At the moment, the president can limit the briefings to a few congressional leaders.

Leading Democrats say that President Bush used that provision to hide covert operations and entire programmes from congressional oversight.

But President Obama has threatened to veto the bill, saying that expanding briefings would undermine ‚Äúa long tradition spanning decades of comity between the branches regarding intelligence matters".

The CIA issued a statement noting that it "took the initiative to notify the oversight committees" about the shortcomings in briefing Congress. It said the agency and Panetta "believe it is vital to keep the Congress fully and currently informed".