The debate over whether Ukraine should be allowed to fire Storm Shadow missiles, made by the UK and France, deep into Russian territory played out for months behind the scenes. The missiles require US technology to be fully effective.
Ukraine is understood to currently only be allowed to use the missiles in limited circumstances on Russian land, such as to hit troops just by the border - though the specifics of any agreements have never been made public.
Mr Zelensky has been calling for permission to fire the missiles further into Russia for months as it could help stop Russian planes taking off before bombing Ukraine.
Well-placed UK government sources said Sir Keir and Mr Macron had become supportive earlier this year.
Previous hopes Mr Biden would approve further use of the missiles were dashed in September over fears of retaliatory attacks on Western military bases.
Antony Blinken, the US secretary of state, was supportive but Mr Sullivan was against the move, fearing it would escalate the stand-off with Russia and further entangle America, according to the Foreign Office’s analysis of internal debates.
Should Kyiv be given permission to fire Storm Shadows deep into Russian territory before Mr Trump took office it would be more difficult for the approval to be reversed.