Ukraine must be in the “strongest possible position” to defend itself, Downing Street has said when asked whether Britain would follow the US in granting Kyiv permission to strike targets in Russia with allied weapons.

Joe Biden has allowed Ukraine to use US-supplied weapons to strike sites in Russia after months of pressure from Ukrainian President Volodymyr Zelensky.

No 10 would not be drawn on whether the UK would follow suit, but did not rule out allowing Kyiv to use British-supplied Storm Shadow missiles.

Our support in Ukraine is ironclad and will continue on that basis. But as to the specifics of the support that we provide … we won’t get into speculation or a commentary on operational discussions

The UK Government has been reluctant to give Ukraine permission without a shift in US policy, in part because the missiles use American navigation data.

The outgoing US president’s decision could see long-range missiles used initially in the Kursk region, where Ukrainian troops have launched an incursion into Russian territory and President Vladimir Putin has bolstered his defences with troops from North Korea.

Downing Street said the UK Government wanted to put Ukraine in the “strongest possible position going into the winter to defend its sovereign territory”.

The UK will “double down” to ensure Ukraine is “provided with the support that it needs for as long as it needs”, a No 10 spokeswoman said, though she refused to say whether that would include allowing Storm Shadow missiles to be fired at targets in Russia.

“Our support in Ukraine is ironclad and will continue on that basis. But as to the specifics of the support that we provide … we won’t get into speculation or a commentary on operational discussions.

“It also remains the case that no war was ever won with one single piece of kit,” she said.

No 10 also suggested that publicly announcing a change in the UK’s position “would only serve to benefit Putin during an ongoing illegal war”.

Asked if it was fair to say the UK would work in lockstep with the US, the spokeswoman replied: “It has always been the case we work closely with our allies and engage to ensure that strategically we are providing the support that Ukraine needs, and we do that in consultation both with Ukraine and with each other as allies.”

Mr Zelensky has been lobbying western allies for permission to strike targets in Russian territory, arguing that he is effectively being forced to fight with his hands tied because he cannot hit the bases Mr Putin is using to launch air attacks against his country.

Mr Zelensky said “long-range capabilities” are a key part of his “victory plan”.

But Moscow said the move would change the nature of western involvement in the conflict.

According to Russia’s Tass news agency, Kremlin spokesman Dmitry Peskov said: “If such a decision has indeed been formulated and communicated to the Kyiv regime, then of course this is a qualitatively new round of escalation of tensions and a qualitatively new situation in terms of the involvement of the United States in this conflict.”

Sir Keir Starmer told reporters travelling to the G20 summit in Brazil that support for Ukraine was “top” of his agenda at the gathering of world leaders.

Mr Biden will be at the summit, while Russia will be represented by foreign minister Sergei Lavrov.

The change in the final months of the Biden administration comes as Kyiv and western allies brace for a possible change of US stance under Donald Trump, who has said he could end the war “in a day”.

The shift in US position could have a greater political than military impact for Ukraine’s forces, partly due to limited supply, but also because Russian strikes are being launched from beyond the 300km (186-mile) range of the US-supplied Army Tactical Missile System (ATACMS) or the UK’s Storm Shadow and its French equivalent Scalp.

Matthew Savill, military sciences director at defence think tank the Royal United Services Institute (Rusi), said: “Many of the best targets – helicopters and glide-bomb armed aircraft attacking Ukrainian cities and troops in the north or in Kursk – have largely moved to air bases outside the range of ATACMS.

“This still leaves plenty of opportunities to strike military headquarters and ammunition or supply locations supporting Russian and North Korean troops, but this would be a reduced impact from when the Ukrainians first requested these weapons.

“The impact may be more political, albeit with a narrowing window of opportunity.

“The Ukrainians need to convince the incoming US administration that they are still worth backing – in President Trump’s transactional view, a ‘good investment’. And they will want to convince him to link his and US credibility to a ‘winning’ outcome, not a major compromise that sees the US ‘lose’.”

A Ministry of Defence intelligence assessment underlined the scale of losses being suffered by the Russians, with 1,498 casualties a day on average in the first 12 days of November, an increase on the most costly month of the war in October when the daily total was 1,354.

“Russia will highly likely continue to experience high casualty rates over the remainder of the year with continued dismounted infantry attacks on multiple axes throughout the winter,” the assessment said.