But No 10 said it was a matter for Mr Trump, as this years host, if he wants to invite the Russian president, Vladimir Putin, to the gathering as a guest.
President Trump said on Saturday that he planned to postpone the annual summit of world leaders until September and he wanted to invite Russia, alongside South Korea, Australia and India, as part of an alliance to discuss future relations with China.
The US president has previously faced resistance from other G7 members including the UK – all of whom hold vetoes on proposed changes to the membership of the body – to the readmission of Russia, which was expelled from what was then the G8 in 2014 over its illegal annexation of Crimea.
But he described Septembers gathering – postponed from next week because of coronavirus – as potentially a “G10 or G11”, raising the prospect of a formal expansion of membership.
Mr Trump characterised the G7, which also includes Canada, Japan, Germany, Italy and France, as “a very outdated group of countries”, adding: “I dont feel that as a G7 it properly represents whats going on in the world.”
The addition of others would create “a nice group of countries”, he suggested.
Asked about Mr Trumps comments, Boris Johnsons official spokesperson left no doubt at a daily Westminster media briefing that the UK was ready to wield its veto to prevent readmission of Russia.
The spokesperson said: “Decisions on G7 membership need to be made unanimously.
“Russia was removed from the G7 group following its annexation of Crimea and we are yet to see evidence of its changed behaviour which would justify readmittance.
“We would not support it being readmitted as a member of the group unless it ceases aggressive and destabilising activity that threatens the safety of UK citizens and the collective security of our allies.
“In terms of invitations issued by the host country, that is a matter for them. At recent G7s, the host country has invited countries from outside the group. Its an established principle that that is what the host nation can do.”