Newt Gingrich — one of President Trump’s strongest allies — said on Monday that Trump’s performance alongside Vladimir Putin in Helsinki was “the most serious mistake of his presidency.”

Gingrich, former GOP speaker of the House and one of the president’s biggest defenders, said Trump’s grievous error was siding with Putin over US intelligence agencies.

“President Trump must clarify his statements in Helsinki on our intelligence system and Putin,” Gingrich tweeted.

“It is the most serious mistake of his presidency and must be corrected—-immediately.”

During a joint press conference with Putin earlier on Monday, Trump disagreed with both US intelligence assessments and a detailed indictment from Special Counsel Robert Mueller that claimed Russian officials conspired to interfere in the US election.

“I have great confidence in my intelligence people, but I will tell you that President Putin was extremely strong and powerful in his denial today,” Trump said.