Ben Wallace spoke to a Russian hoaxer for nearly 10 minutes because Home Office officials had vouched for the individual, Whitehall sources have claimed.

The Defence Secretary was assured the impostor was legitimate by the department’s officials, as a government blame game erupted across Whitehall.

Mr Wallace, in Poland at the time of the “dirty tricks” operation, believed he was speaking to Denys Shmyhal, Ukraine’s prime minister.

The pair spoke for around nine minutes before Mr Wallace became suspicious at the nature of the questions, which went “way beyond” the security classification for the phone line.

The issue raises questions around the vulnerability of key government officials at a crucial time in the Ukraine war.

A cross-government inquiry will seek to establish how Whitehall security protocols could have been breached so easily.   

The Defence Secretary said that the man, who was sitting in front of a Ukraine flag, looked and sounded like Mr Shmyhal, but admitted the pair had never met in person.