Britain will not go it alone and let Ukraine use long-range missiles against targets in Russia without the backing of the United States, The Times reports.
Five former defence secretaries and former Prime Minister Boris Johnson have urged Britain to allow the use of Storm Shadow missiles without American support.
And Ukrainian President Zelenskyy has called on the UK and US to move past their fear and act with decisiveness as momentum slows in Kursk and Belgorod.
But Britains action is still bound by its US guidance systems, judged crucial to ensuring the missiles hit their targets, The Times reports.
The UK nonetheless expects the US is likely to support its backing of Ukraine with a decision expected next week, and the Foreign Secretary insisted over the weekend that Putins threats of nuclear war with the US and NATO would not deter action.
Storm Shadow missiles are seen attached to the hard points of a Eurofighter Typhoon
Foreign Secretary Mr Lammy has been part of intensive UK-US diplomatic talks in Kyiv and Washington in the past week which have so far failed to produce a decision.
Ukrainian servicemen drive Soviet-made T-64 tanks in the Sumy region on August 11
Ukrainian servicemen fire mortars towards Russian positions in the Donetsk region, Ukraine
Its important that as allies supporting Ukraine, we have a shared strategy to win going forward, Mr Lammy told Laura Kuenssberg on BBC1 on Sunday.
Weve been discussing this with the United States and with other key allies, but of course, we head to the UN general assembly, where we will all meet with in just under 12 days time.
Despite opposition within the United States, it is expected the US will back the plan at the UN general assembly in New York next week.
Keir Starmer and Joe Biden held talks in the Blue Room of the White House on Friday to discuss strategy on how to bring the war in Ukraine to an end, including whether to hand over Storm Shadow missiles to Zelenskyy.
Disappointingly for both parties, John Kirby, a spokesman for the US national security council, announced afterwards that there would be no change to our policy on letting Ukraine use long-range missiles.
Vladimir Putin also warned the West would be at war with Moscow should this change, with many of his air bases and logistics hubs vulnerable to Ukrainian attacks if the restrictions were lifted.
According to US sources, Mr Biden has not yet made a final decision.
Despite Vladimir Putins threats that use of the weapons would be tantamount to direct action against Russia from the UK, former defence officials and an ex-Prime Minister have urged the government to press on with the plan.
Boris Johnson urged Sir Keir Starmer to break the impasse, warning that every days delay is costing lives.
Mr Johnson said after a visit to war casualties in Kyiv: It is heartbreaking to visit wounded veterans in recovery centres – as I have in Kyiv this weekend – where Ukrainian heroes are being treated for the injuries they have suffered in the past few months, and to think that some of these casualties could have been avoided if we had given Storm Shadow permissions earlier.
There is no conceivable case for delay. The only person who fears escalation is Vladimir Putin, and every day that goes by is a lost opportunity to save lives and bring about a just conclusion to this war.
In his most outspoken comments of the conflict, Mr Zelensky asked whether Western politicians actually wanted Ukraine to win.
He posted on X on Friday: It is difficult to repeatedly hear, "We are working on this" while Putin continues to burn down our cities and villages. He doesnt need anyones approval.
Anyone who looks at a map clearly understands why Ukraine needs long-range capabilities. Russian logistics and military sites are safe because we cannot reach them.
We know where they are located. All of this can be destroyed in months. So what is missing to end it? Strength? Decisions? Or the will?
Under the Tories, Lord Cameron - then Foreign Secretary - suggested it was up to Ukraine to determine how its missiles were used.
He said in May: In terms of what the Ukrainians do, in our view, it is their decision about how to use these weapons, theyre defending their country, they were illegally invaded by Putin and they must take those steps.
We dont discuss any caveats that we put on on those things. But lets be absolutely clear, Russia has launched an attack into Ukraine and Ukraine absolutely has the right to strike back at Russia.
French President Emmanuel Macron even entertained the possibility of sending French troops to Ukraine to help defend the country if needed.
Soon after, Russia reopened its counter-offensive in Kharkiv.
The assault was Russias biggest gain in 17 months, the Telegraph reported, swallowing up 250sq-km.
Russian troops made it six miles before Ukraine stabilised the situation.
Russian president Mr Putin has warned that allowing long-range strikes would mean that Nato countries, the United States, and European countries are at war with Russia.
Prime Minister Keir Starmer and David Lammy, Foreign Secretary speak as they prepare for a meeting with US President Joe Biden on Friday
Ukrainian servicemen operate a tank on a road near the border with Russia, in the Sumy region of Ukraine, on August 14, 2024
The Foreign Secretary said talks were continuing with the US and allies about giving Kyiv permission to use UK-supplied Storm Shadow missiles to hit Russian airbases and military sites which are being used to launch attacks on Ukraine.
The sudden incursion invited the US to concede the restrictions it had placed on only using its supplied weapons on Russian targets within Ukraine.
The hallmark of our engagement has been to adapt and adjust as necessary, to meet whats actually going on on the battlefield, to make sure that Ukraine has what it needs, when it needs it, US Secretary of State Antony Blinken said at the time.
So far, the US has allowed Kyiv to use American-provided weapons only in a limited area inside Russias border with Ukraine.
Soon followed Ukrainian attacks with foreign missiles over the border, drawing outrage from Putin and his cronies - and threats of nuclear annihilation towards Ukraines western backers.
Ukraine was able to launch an offensive back into Russia in early August, taking scores of villages in Kursk and Belgorod oblasts.
But momentum has waned in recent days, with Russia mobilising to halt the attack.