Professor Neil Ferguson has resigned from the government's scientific advisory group (SAGE) after reports he broke coronavirus lockdown rules.
The leading epidemiologist from Imperial College said he had made an "error of judgement" in a statement announcing his departure.
As a prominent member of SAGE, his resignation represents a blow for the group and ministers he is helping guide policy around the pandemic.
"I accept I made an error of judgement and took the wrong course of action," he said.
"I have therefore stepped back from my involvement in SAGE.
"I acted in the belief that I was immune, having tested positive for coronavirus and completely isolated myself for almost two weeks after developing symptoms.
"I deeply regret any undermining of the clear messages around the continued need for social distancing to control this devastating epidemic.
"The government guidance is unequivocal, and is there to protect all of us."
While SAGE has around 50 members, Professor Ferguson had become one of the most prominent after featuring in some of the the government's regular coronavirus briefings.