Ukraine may be given private dispensation by the UK and US to fire long-range Storm Shadow missiles into Russia without making it public until the first one is fired.
Western diplomats are poised to sign off on the decision but could hold back from making an official announcement at the UN next week.
The Times understands there is wariness about publicising the move in advance to alert Russia.
An agreement could therefore be struck in private to allow Ukraine to fire the British and French-made cruise missiles into Russia with the element of surprise, diplomatic sources said.
The UK and France have already sent Storm Shadow missiles to Ukraine, but only for use on targets within the country’s borders to fight back Russian invading forces.
However, Ukraine has been pushing for authorisation to use the missiles to strike deep inside Russia, targeting ammunition stores and bunkers that are harder to penetrate with drones.
Officials had been preparing to sign off on the move after a joint US-UK trip to Ukraine last week, but the decision was on such a knife edge that two statements for their joint press conference were prepared.
One was drafted to announce the decision to authorise Ukraine’s use of the missiles, and the other, which was delivered, said they were still considering the request.
David Lammy, the foreign secretary, this week held talks in Paris with Antony Blinken, the US secretary of state, as well as foreign ministers from France, Germany and Italy.
The allies worked to thrash out a deal ahead of the UN general assembly next week, where Sir Keir Starmer is heading for talks with other world leaders.
Lammy said the talks in Paris on Thursday were about ensuring that “Ukraine has all it needs, militarily, politically, diplomatically and in terms of aid to get through what will be a tough winter and into 2025”.
Pressed on whether Britain was poised to authorise the use of Storm Shadow missiles, Lammy said: “I haven’t got further announcements to make today.”
He added that discussions were continuing and that “we look forward to meeting with President Zelensky in a few days time at the UN general assembly”.
UK diplomatic sources said they were keen to show Nato was “moving as one”, and avoid the impression of Britain and the US “dominating decisions”.
However, one official suggested senior figures in the US administration were focused on “legacy-building” and wanted to move faster, given the presidential election in November.
Starmer met President Biden in Washington DC last week, further raising expectations that the authorisation for Ukraine could come within days.
British officials believed Blinken had signalled the US was moving closer giving the green light. However, Biden’s team were said to be holding out for Zelensky to present a “victory plan”.
Boris Johnson, the former prime minister, and five former Tory defence secretaries have urged Britain to go it alone and authorise the use of the Storm Shadow missiles if needed.
• What would western long-range missiles mean for Russia-Ukraine war?
Ben Wallace said that failure to move now would make Britain “appeasers” of the Kremlin, while Gavin Williamson called it a “dereliction of duty”. Johnson added: “There is no conceivable case for delay.”
Pressure has been growing for Ukraine to be allowed to fire Storm Shadow missiles into Russia since it was claimed the Kremlin had received new deadly ballistic missiles from Iran.
A government official said: “There has been no change in the UK’s position. We continue to provide military aid to support Ukraine’s clear right of self-defence against Russia’s illegal attacks in accordance with international humanitarian law. We are clear that equipment provided by the UK is intended for the defence of Ukraine.”
