WASHINGTON — Sparks flew Wednesday as Secretary of State John Kerry sparred with GOP lawmakers who sent a letter to Iranian leaders stating Congress would have to approve any nuclear agreement.

“It’s really almost insulting that the presumption here is that we’re going to negotiate something that allows them to get a nuclear weapon,” Kerry told Sen. Marco Rubio (R-Fla.) at a Senate Foreign Relations Committee hearing.

Kerry, who once chaired the prestigious committee, has been at the helm of nuclear-program negotiations involving five countries and Iran.

He lectured his former colleagues for more than five minutes on how the “stunning” letter signed by 47 GOP senators undermined foreign affairs and was “absolutely incorrect” in assuming Congress can undo an executive-to-executive agreement.

“The senators’ letter erroneously asserts that this is a legally binding plan. It’s not,” Kerry said. “Number two, it’s incorrect when it says that Congress could actually modify the terms of an agreement at any time. That’s flat wrong.”

Sen. Bob Corker (R-Tenn.), the committee chair, finally cut off Kerry.

“Mr. Secretary, I know this is a well-written speech, but you’ve been at this for five minutes.”

“It’s not a speech,” Kerry retorted. “This is a statement about the impact of this irresponsible letter.”

“I didn’t sign the letter,” said Corker, one of the few Republicans who refused to join. “I’m very disappointed, though, that you’ve gone back on your statement that any agreement must pass muster with Congress.”

Sen. Rand Paul (R-Ky.) took his criticism further, saying he’s “not particularly happy with being lectured to” since the Obama administration “has trampled the Constitution at many turns.”

“I signed the letter to Iran,” Paul said. “But you know what, the message I was sending was to you. The message was to President Obama. We want you to obey the law.”