According to the Foreign Office in London, the diplomats' accreditation was withdrawn in August.
The security service FSB claims that the diplomats belong to a unit within the foreign administration whose main task is to inflict "strategic losses" on Russia. They are also alleged to have gathered intelligence and engaged in "subversive activities".
The six are also said to have had meetings with independent media and human rights groups, thereby being labeled as "foreign agents". This designation is generally used by President Vladimir Putin's regime to silence and restrict dissenting voices.
The news about the diplomats comes at the same time as the UK's Prime Minister Keir Starmer visits the US capital Washington DC. A meeting with President Joe Biden is on the agenda, where, among other things, Ukraine's request to use long-distance weapons donated by the West against targets deep within Russia will be discussed.
Putin has said that such a development would mean that his country is at war with the defense alliance NATO.