The White House’s top lawyer has told House Republicans to give up on their impeachment inquiry into Joe Biden, calling the investigation a “charade”.

The White House counsel, Ed Siskel, told the House speaker, Mike Johnson that “it is clear the House Republican impeachment is over” in a scathing letter sent on Friday morning.

Siskel said that despite House Republicans collecting over 100,000 pages of records and conducting interviews with dozens of witnesses, including multiple public hearings, “none of the evidence has demonstrated that the president did anything wrong”.

“This impeachment inquiry in large part has been based on allegations made by troubling sources,” Siskel wrote, referring to an FBI informant who was criminally charged last month for lying about his explosive allegations about Biden.

Siskel’s four-page letter is a sign that the White House is taking an increasingly aggressive stance against the impeachment inquiry, which Democrats say is a clear effort to distract voters during an election year.

“It’s obviously time to move on,” he wrote. “There is too much important work to be done for the American people to continue wasting time on this charade.”

Early in the letter, Siskel tells Johnson that members of his own party are starting to turn away from him because of the impeachment inquiry. He cites reports that Republicans are looking for an exit strategy after failing to clinch the impeachment.

Siskel also quotes Ken Buck, a Republican representative from Colorado who announced earlier this week that he would resign from his post at the end of March, leaving the GOP with a weaker House majority.

“We’ve taken impeachment and we’ve made it a social media issue as opposed to a constitutional concept. This place keeps going downhill and I don’t need to spend more time here,” Buck said.

House Republicans over the last six months have been trying to find evidence that Biden benefited from his son Hunter Biden’s foreign business activities. But after making clear their goal of impeaching the president, Republicans are scrambling to determine how to move forward.

The House oversight committee is holding a hearing next week with former business partners of Hunter Biden, but it is unclear whether witnesses can deliver the definitive evidence Republicans have been hoping for.

A spokesperson for Johnson on Friday accused Biden of lying and said it was not up to the White House to decide what happened with the inquiry.

“The White House does not get to decide how impeachment gets resolved. That is for Congress to decide,” Raj Shah said in an email.

Johnson himself has said previously that “people have gotten frustrated” with the inquiry. Though he “has not been able to take the time to do the deep dive into evidence”, he insists that what has been revealed is “alarming”.

“I know that people have gotten frustrated sometimes that it’s [dragged] on too long. But in our constitutional system, that is the way it’s supposed to work,” Johnson told reporters on Wednesday.

Reuters contributed reporting