The Keegan Theatre really should provide seatbelts.

Remaining still and seated during a performance of Keegan’s revival of the comic gem of “Noises Off!” is next to impossible.

A clear audience favorite, by act two of this three-act comedy, which Frank Rich of the New York Times has called “the funniest play written in my lifetime,” the packed house was gripping their armrests to keep from spilling out of their chairs. And the intimate setting of the Church Street Theatre echoed with repeated rounds of titters and belly-laughs, hoots and guffaws.

This knockabout tour-de-farce is in fact farce of the first degree, with slamming doors and pants off and dresses down, with props and quips and cast members leaping about the stage. And its run has just been extended through Aug. 29.

So now you simply have no excuses left for missing this one, a 1982 play by English playwright and novelist Michael Frayn, one of the handful of writers in the English language to excel in both drama and prose fiction, who often uses humor and horseplay to raise philosophical questions — his Cambridge degree is in philosophy.

But don’t see “Noises Off!” expecting metaphysics. Go expecting a dreadful sex farce, staged as a play-within-a-play — the utterly awful “Nothing On” — the type of English music-hall play where comely girls run about in their underwear, old men (and young) drop their trousers, and the many doors continuously open and close as the manic menagerie of second-rate actors try their best to get through the first act.

But with every stage step the cast — uniformly funny in an ensemble slapstick performance that will surely be a nominee for a Helen Hayes award — delivers superbly executed, fast-paced timing of jokes, droll double-entendres and burlesque pratfalls.

In theatrical stage directions, the term “noises off” specifies sounds meant to originate offstage. In the hands of Keegan’s artistic director Mark Rheas, at the helm of “Noises Off!,” the continuous whirl of hilarious action is both off and onstage. Each of Frayn’s three acts contains a performance of the first act of “Nothing On,” each act separated by weeks as the very ragged touring theatrical troupe verges further and further with each stage misstep into comic chaos.

Act one opens with the final dress rehearsal prior to opening night, with the seedy cast still forgetting lines, missing cues and mishandling props. Even worse, cast eccentricities and personal problems, including two different love triangles, haunt backstage relationships, and jealousy and petty squabbling overwhelm them, but always to huge comic effect.

But the tidal wave of comic miscues and mishaps begins to really swell with act two — as the motley crew still fumbles with missed entrances and exits, and prop problems, including the telephone and especially several plates of sardines.

By act three, we see a performance of “Nothing On” near the end of its 10-week run as personal frictions have grown raspier and off-stage shenanigans and onstage bedlam reach their crescendo in dreadfully bungled performances (seen from backstage).

Singling out cast members for special recognition is risky when all are so superb, but special recognition should go to Brianna Letourneau as deliciously clueless actress Brooke Ashton (or “Vicky” in the sex farce), alluring in her finely turned derriere and heaving bosom.

This classic comedy — a hit with critics and audiences everywhere and even in a 1992 film version starring Michael Caine and Carol Burnett and Christopher Reeve — will leave you laughing out loud for days afterward.