Sir Keir Starmer plans to thwart Donald Trump on Ukraine policy

The debate over whether Ukraine should be allowed to fire British- and French-made Storm Shadow missiles deep into Russian territory played out behind the scenes for months. For the missiles to be fully effective, US technology is required.

It is understood that Ukraine is currently only allowed to use the missiles on Russian soil in certain circumstances, such as to hit troops directly on the border – but the details of any arrangements have never been made public.

Mr. Zelensky has for months demanded permission to fire the missiles further into Russia, saying it could help stop Russian planes from taking off before they bomb Ukraine.

Well-informed British government sources said Sir Keir and Mr Macron had moved towards support earlier this year.

Earlier hopes that Mr. Biden would agree to the continued use of the missiles were dashed in September amid fears of retaliatory attacks on Western military bases.

Antony Blinken, the US secretary of state, supported the move, but Mr Sullivan opposed the move, fearing it would escalate the standoff with Russia and further embroil America, according to a Foreign Office analysis of internal debates.

Should Kiev receive permission to fire Storm Shadows deep into Russian territory before Mr. Trump takes office, it would be harder to reverse the authorization.