The debate over whether Ukraine should be allowed to fire Storm Shadow missiles, made by the United Kingdom and France, deep into Russian territory has played out for months behind the scenes. Missiles require American technology to be fully effective.

Ukraine is currently only allowed to use these missiles in limited circumstances on Russian territory, such as to strike troops right on the border – although details of the agreements have never been made public.

Mr. Zelensky has for months been seeking permission to fire missiles further into Russia because it could help prevent Russian planes from taking off before bombing Ukraine.

Well-placed British government sources said Sir Keir and Mr Macron became supportive of the project earlier this year.

Previous hopes that Mr. Biden would approve continued use of the missiles were dashed in September. for fear of reprisals on Western military bases.

Antony Blinken, the US secretary of state, favored the move, but Mr Sullivan opposed it, fearing it would deepen the standoff with Russia and further entangle America, the analysis found internal debates at the Ministry of Foreign Affairs.

If kyiv were authorized to fire Storm Shadows deep into Russian territory before Mr. Trump took office, it would be more difficult to reverse that approval.