Five former defence ministers and Boris Johnson have urged Sir Keir Starmer to allow Ukraine to use Storm Shadow missiles against Russia, even without US backing.
Grant Shapps, Ben Wallace, Gavin Williamson, Penny Mordaunt, Liam Fox and Johnson united in warning Starmer that further delay would only strengthen Putin's resolve.
Starmer's recent talks with Biden in Washington ended without agreement on the issue.
Grant Shapps told The Sunday Times: "Rather than waiting for formal approval from the US, Sir Keir needs to provide President Zelensky with what's desperately needed today. That's how we assumed our global leadership position in supporting Ukraine."
Ben Wallace warned: "Britain is in danger of falling behind into the pack of ditherers, appeasers and delayers, when the only real way to stand up to a bully such as Putin is to be strong, united and determined to see it through."
The call comes amid reports of Russia receiving new ballistic missiles from Iran, possibly in exchange for nuclear secrets.
Gavin Williamson added: "From the start of this war Britain has not hesitated to take a lead to step forward when others have hesitated. Starmer needs to show that same leadership and determination. Failing to do so would be a dereliction of his duty."
Boris Johnson stated: "There is no conceivable case for delay."
The debate follows similar delays over decisions to supply tanks and fighter jets to Ukraine.
Wallace emphasised Britain's leading role in supporting Ukraine: "We were the first in Europe to put in lethal weapons and we have continued to support Ukraine all the way through."
It comes after Ukraine renewed calls on Saturday for the West to allow it to use missiles to strike deeper into Russia after a US-UK meeting failed to reach an agreement on the issue.
The renewed appeal came as Kyiv said Russia launched more drone and artillery attacks into Ukraine overnight.
"Russian terror begins at weapons depots, airfields and military bases inside the Russian Federation," Ukrainian presidential adviser Andriy Yermak said Saturday.
"Permission to strike deep into Russia will speed up the solution."
So far, the US has only allowed Kyiv to use American-provided weapons only in a limited area inside Russia's border with Ukraine.
British Prime Minister Keir Starmer met US President Joe Biden at the White House on Friday, with the possibility of Kyiv using long-range missiles against targets in Russia thought to have been discussed.
But Western leaders have yet to signal a decisive policy change as Russian leader Vladimir Putin warned of an escalation in the war if the move should go ahead.
A senior Nato official on Saturday said there were good military reasons to allow Ukraine to use the long-range missiles.
Admiral Rob Bauer said the law on armed conflict gave a nation the right to defend itself and that did not stop at its border - but that those providing weapons also had the right to put restrictions on their use.
He said: "In military terms, you do (those attacks) because you want to weaken the enemy that attacks you in order to not only fight the arrows that come your way, but also attack the archer.
"So, militarily, there is a good reason to do that; to weaken the enemy, to weaken its logistics lines, fuel, ammunition that comes to the front.”
Questioned on what they had decided in relation to Ukraine's potential use of long-range missiles, Sir Keir earlier told reporters: "We had a wide-ranging discussion about strategy in Ukraine, of course, in the Middle East and other parts of the world.”
Concern about escalation has been one of the reasons why permission has not yet been given to Kyiv for the unrestricted use of Western weapons.
Iran has been hit with sanctions by the UK and US after the two countries formally accused Tehran of supplying ballistic missiles to Russia.
On Saturday, the foreign ministers of the Group of the Seven condemned "in the strongest terms" Russia's procurement of Iranian ballistic missiles.
"Iran must immediately cease all support to Russia's illegal and unjustifiable war against Ukraine and halt such transfers of ballistic missiles, UAVs and related technology, which constitute a direct threat to the Ukrainian people as well as European and international security more broadly," the G7 ministers said in a statement.
G7 is made of Canada, France, Germany, Italy, Japan, Great Britain and the U.S.
Earlier this week, British foreign minister David Lammy and US Secretary of State Antony Blinken announced further financial support for Ukraine, including a £600 million package from the UK and 717 million dollars (£550 million) from the US to meet immediate humanitarian, energy and stabilisation needs.
The UK package includes a reaffirmation of Rishi Sunak's pledge of £242 million, as well as 484 million dollars (£371 million) worth of loan guarantees for World Bank lending before the end of the year, while the US package includes 325 million dollars (£250 million) to support Ukraine's energy needs.
