Britain's government rects any idea of readmitting Russia as a member of the G7, but would not stop it from attending as a guest, at any summit, a spokesman for Prime Minister Boris Johnson said on Monday.

U.S. President Donald Trump said on Saturday he would postpone a Group of Seven summit he had hoped to hold next month until September or later and expand the list of invitees to include Australia, Russia, South Korea and India.

Johnson's spokesman said the prime minister would wait to see what the United States proposed.

"We will look at the detail of what the U.S. is proposing. It is customary for the country that holds the G7 presidency to invite other leaders to participate as guests in the summit," he told reporters.

"Russia was removed from the G7 group of nations following its (2014) annexation of Crimea and we are yet to see evidence of changed behavior which would justify its readmittance ... We wouldn't support it being readmitted as a member of the group."