The embattled director of the FBI has been accused of covering up evidence of Donald Trump’s links to Russia while inflicting severe damage on Hillary Clinton, as Democrats hit back in a growing scandal involving her email server.

Harry Reid, the Democrat leader of the Senate, accused James Comey of “a disturbing double standard” and, in a remarkably forthright letter, said he regretted supporting a man who he once believed was “a principled public servant.”

“I now see I was wrong,” he wrote, in a letter to Mr Comey on Sunday.