A recent hearing of the Senate Intelligence Committee. (Harry Hamburg — Associated Press)

Cuts that go too far “will compromise missions such as strategic warning, counterterrorism activities, counterintelligence, and cybersecurity,” the committee said in a letter sent to the Joint Select Committee on Deficit Reduction, which has been getting more than its fair share of advice lately.

Steep reductions could also backfire, the intelligence panel said, leading “to higher costs in recovery or response due to the failure to prevent a terrorist attack or avoid unnecessary conflict.”

The committee didn’t make the connection explicit, but was clearly alluding to the massive federal spending over the past decade — much of it triggered by the failure to prevent the Sept. 11 attacks and the mistaken belief that Iraq had stockpiles of banned weapons.

So how much can the CIA and other spy agencies afford to cut? Neither the committee nor the nation’s intelligence chief has offered a detailed plan. But both seem to be circling the same scenario: spending reductions that would add up to $10 billion or more over the next decade, but only amount to slight decreases year-to-year.

Asked to outline the likely end-game, a senior congressional aide said, “That’s sort of the million -- or billion -- dollar question. I think we’re talking about an actual cut below the current levels of a few percent each year.”

Reductions at that modest a level, the official said, would allow Director of National Intelligence James Clapper to deliver on his pledge this week to achieve “cuts in the double-digit range -- with a B -- over ten years.”

Even so, the reductions would require painful adjustments for the intelligence community, where spending on spy agencies that aren’t part of the armed services has doubled over the past decade to roughly $55 billion. That growth has helped prevent follow-on terrorist attacks, but has also been accompanied by waste.

“The magnitude and speed of the growth in intelligence budgets did ... result in programs and activities that were not always fully coordinated or integrated,” said the letter, which was signed by Sen. Dianne Feinstein (D-Calif.), chairman of the committee, as well as Saxby Chambliss of Georgia, the top Republican.

House members have also weighed in. Rep. Mike Rogers (R-Mich.), the chairman of the House Intelligence Committee, submitted a letter to the so-called “supercommittee” noting that budget cuts in the 1990s weakened intelligence services significantly in the years leading up to the Sept. 11 attacks, necessitating the “vast increases in public expenditures” that followed.