In a plot twist that even Hollywood couldn’t dream up, Congressional leaders are on the verge of banning George Clooney and the producers of HBO’s upcoming “K Street” series from filming in the Capitol or in the offices of Members of Congress.

The decision, which will be made public in a letter as early as today, throws the first bureaucratic barrier in the path of the hyped docudrama intended to capture on camera how real-life lobbyists use their political muscle to remove just such obstacles.

“It would be very chaotic if we had film crews set up all over the place,” said Sen. Trent Lott (R-Miss.), the head of the Rules and Administration Committee.

The Senate Rules and Ethics committees are drafting a joint letter barring the cast and crew from filming on Capitol Hill, aides confirmed. The letter will refer specifically to the new HBO show and cite a 20-year precedent of restricting commercial filming on Capitol Hill.

“The rules are very specific about commercial filming on Capitol grounds,” Lott added, referring to a section of the U.S. Code.

It is unclear if the House is mounting a similar effort, though most of the filming so far has taken place in the Senate.

An HBO spokeswoman acknowledged that Clooney and his crew were made aware of the rule Wednesday and agreed to stop filming.

The ruling comes just days after the show’s cast and crew arrived in Washington and began filming for this Sunday’s season premiere.

But the show’s producers say it will have little impact on their work.

“We know the rules and it is no big deal,” said GOP consultant Stuart Stevens, one of several well-known Washington figures co-producing the 10-part series.

Democratic strategist James Carville and Republican media strategists Mary Matalin and Michael Deaver also are lending a hand to producers Clooney and Steven Soderbergh.

An HBO spokeswoman also downplayed the decision, saying that the crew will simply “shoot off of the Capitol grounds.”

At the very least, however, the crew has lost some valuable time — and a few hours of footage — as they scramble to put together the season’s first episode in time for Sunday’s premiere.

But the decision could be more significant for the show.

The unscripted, largely improvised series intends to give viewers a behind-the-scenes glimpse at how Washington works by using hand-held cameras to document lobbyists from a fictitious K Street firm as they interact with real-life Members of Congress and powerbrokers.

Some of the most compelling scenes in the 10-minute pilot of the show were shot in the Senate.

In one memorable moment, actress Mary McCormack (“K-PAX,” “Full Frontal”) tells Sen. Orrin Hatch (R-Utah) that she is a Republican during a meeting in the Senator’s Hart Building office.

“Good,” Hatch responds, before adding: “You didn’t have to be to get in here, but I recognize your firm and I think you do a really good job.”

Later in the test episode, McCormack makes impromptu pitches to Sens. Hillary Rodham Clinton (D-N.Y.) and John McCain (R-Ariz.) after running into them in Senate hallways.

Before Senate leaders delivered the word to Clooney, the HBO cast and crew had spent several hours on Capitol Hill talking to Senators and lining up other meetings.

On Tuesday, Clooney shot McCormack meeting with Sen. Rick Santorum (R-Pa.) in his office. Santorum serves as the Senate GOP’s real-life liaison to the K Street community.

The two talked about Democratic presidential politics, Penn State football and other matters before Santorum asked McCormack to help him round up support for a charitable giving bill that Santorum is currently championing in the Senate.

Santorum said he was instructed to “say whatever you would normally say or do whatever you normally would do if this situation was presented to you.”

Said Santorum: “Well, I said if somebody came in and was hitting me up on something, I would probably say I would be happy to help, but now you can help me on something.”

Santorum said he had no set lines. “I just played it freewheeling.”

After McCormick pleaded with the Senator to support her client’s legislation, Santorum asked the faux K Street lobbyist for her help.

“They said do whatever you would normally do,” Santorum recalled. “I said, ‘Well, what kind of clients does this person represent — what do they do?’ Right now if somebody came in with her set of clients, I would talk to them about the Charitable Giving Act, which I am trying to get passed and I am having some problems with the corporate community in complaining about some of the offsets to the faith-based bill.”

He continued: “I mentioned that to her as I would normally. I had [Chamber of Commerce CEO] Tom Donohue in my office earlier in the day and I mentioned that to him. So, I said that is what I did this morning and that is what I would probably do here. So that is what I did. She said, ‘Well, I will talk to some people at [the National Association of Manufacturers],’ and that was it.”

Under the decision, Clooney and the crew would lose authentic scenes like the one captured in Santorum’s office.

Clooney also planned to shoot Sen. Don Nickles (R-Okla.) this week, but it is now unclear if that will happen.

Overall, “K Street” will likely suffer few long-term effects from the decision.

Indeed, much of the first episode appears to focus on former Vermont Gov. Howard Dean (D) and the Democratic presidential candidates.

On Tuesday, Clooney and actor Roger Guenveur showed up with their camera at the Democrats’ presidential debate in Baltimore — where they were mobbed by real-life reporters, cameramen and sound technicians as they tried to graze on a pre-debate buffet.

On Capitol Hill, the three-member cast — McCormack, Guenveur and John Slattery — has moved around much easier than their famous producer.

The actors went nearly unnoticed on the first floor of the Senate when waiting for a lunch guest to arrive on Wednesday afternoon.

None of the Senate aides and pages who hurried past them so much as glanced at them — and at least one newspaper photographer assigned to shoot them let the trio slip because he could not recognize them.

Mark Preston contributed to this report.