The FBI has acquired a warrant to investigate emails found on a laptop used by an aide to Hillary Clinton as part of its investigation into the Democratic presidential nominee’s use of a private email server.

The move came as senior Senate Democrats made an extraordinary attack on the head of the FBI, James Comey, on Sunday over the new investigation, with the Senate minority leader, Harry Reid, warning he may have broken the law.

In a scathing letter, Reid wrote: “Your actions in recent months have demonstrated a disturbing double standard for the treatment of sensitive information, with what appears to be a clear intent to aid one political party over another.

“My office has determined that these actions may violate the Hatch Act, which bars FBI officials from using their official authority to influence an election. Through your partisan action, you may have broken the law.”