Vladimir Putin has been accused of trying to distract attention from his “heinous” atrocities after Russia claimed the UK and France are plotting to supply Ukraine with a nuclear weapon or “dirty bomb”.

On the fourth anniversary of the Russian president’s full-scale invasion of Ukraine, Sir Keir Starmer hit out at Putin for blocking the path to peace and vowed to stand with Kyiv for as long as it takes.

He said it was a “falsehood” to suggest Russia is winning the conflict, which is costing Putin’s forces tens of thousands of lives every month.

Meanwhile Downing Street said on Tuesday there is “no truth” to Russian claims that Britain is planning to supply Ukraine with a dirty bomb.

The Ministry of Foreign Affair of Russia made the bizarre claim that British and French “elites are actively exploring ways to supply Kyiv with a nuclear bomb, or at least a so-called dirty bomb” on Tuesday.

Allegations that Ukraine was preparing to use an explosive device laced with radioactive material were also made by the Kremlin to the UN Security Council in 2022, when they accused Ukraine of preparing to detonate a bomb in order to accuse Moscow of using weapons of mass destruction.

Asked about the latest claims, the Prime Minister’s official spokesman described them as “a clear attempt by Vladimir Putin to distract” from his latest actions in Ukraine.

“There is no truth to this,” the spokesman said.

He added: “You will have seen the Prime Minister’s words this morning, paying tribute to the incredible resilience of the Ukrainians, and you will have seen the package of action on military, humanitarian, reconstruction fronts announced today, which is a sign of where our support is for Ukraine. We will continue with our efforts to secure a just and lasting peace.”

Sir Keir said Putin “will not win” his brutal war in Ukraine, as fresh sanctions on Russia were revealed.

Ahead of a coalition of the willing meeting alongside French President Emmanuel Macron on Tuesday, the Prime Minister added that Britain would be “stepping up” military and humanitarian help for Ukraine.

Britain has also chosen the fourth anniversary of the full-scale invasion to slap fresh sanctions on Russia, with the aim of squeezing its oil and gas industry to limit funds for the war.

The package, which is the largest imposed since the early months of the war in 2022, includes pipeline company PJSC Transneft, which the Foreign Office said was responsible for transporting more than 80% of Russian oil exports, along with 175 companies and 48 tankers involved in “shadow fleet” operations to move Russian oil.

Sir Keir said: “On this grim anniversary, our message to the Ukrainian people is simple: Britain is with you, stronger than ever. That is why we are announcing new support today and we will continue to support Ukraine for as long as it takes.

“For all the noise in world affairs today, this war remains the most critical issue of our age. It asks the question of whether Ukrainian and European freedom will endure.

“Our answer, together, is unequivocal: Russia is not winning this war. They will not win this war. Ukraine’s courage continues to hold the line for our shared values, in the face of Putin’s aggression.

“We will stand by their side, until a just and lasting peace – and beyond. Slava Ukraini.”

Britain’s Chief of the Defence Staff, Sir Richard Knighton, also stressed on Tuesday that a Russian victory in Ukraine was “far from inevitable”.

Praising the courage of the Ukrainian people, he said: “Through all the suffering imposed by Putin on innocent civilians in pursuit of his illegal war, the Ukrainian people have come together to defend their home.

“They have proved that Russian victory is far from inevitable.”

Russian has suffered over 1.2 million casualties since February 24, 2022, Sir Richard said.

The Russian Black Sea fleet had been “neutered by a country without a navy,’ he added, and “despite overwhelming odds” Ukrainian airspace was still being contested.

“Russia’s war on Ukraine has gone on longer than its involvement in World War II but the frontline has hardly moved since 2022,” Sir Richard emphasised.

It comes as Western Officials said they believe Putin is no longer able to recruit Russian troops quickly enough to replace those dying on the battlefield in Ukraine.

However, armed forces minister Al Carns insisted that the depletion of Russia’s number of fighting men did not mean the UK could relax its commitment to raise defence spending.

Between 30,000 and 35,000 Russian soldiers are recruited each month, according to western officials.

But the number of those killed over the past three months is believed to have been higher than those who joined up.

This means that for the first time in the four years of war, for a sustained period, Moscow’s battlefield numbers are falling.

Mr Carns described this as a “casualty uptick which is disproportionate in scale”.

Western officials believe this will have a significant impact on the Kremlin’s ability to “generate offensive power” for a spring or summer offensive, meaning that any Russian forward march would be sluggish.

It also raises the prospect of “coercive” mobilisation within Russia, such as conscription, rather than relying on the promise of large financial rewards to recruit soldiers.

Drone attacks continue to be the driving force behind the casualty rate on the battlefield, and officials believe that ground-based drones – so-called unmanned ground vehicles (UGVs) – will become ever more present over the coming year.

However, Mr Carns said individual soldiers remain “essential” for capturing and holding ground once drone attacks are over.

As the full-scale war in Ukraine enters its fifth year, the Government has announced a new package of support aimed at bolstering Kyiv’s war effort.

This includes a £20 million package to help repair and protect the nation’s power grid – often a target of Russian strikes – as well as £5.7 million assistance for Ukrainians whose towns and villages are on the front line of the war.

Ukrainian military medics are also being mentored by British surgeons, nurses and physiotherapists in battlefield surgery.

Sir Keir will convene a virtual meeting of the coalition of the willing to discuss recent peace talks and further support for Kyiv amid the ongoing conflict.

Russia’s bombardment of Ukraine continued over the weekend, with a barrage of missiles and drones killing one person in Kyiv and striking the country’s energy grid.

The country’s president, Volodymyr Zelensky, has told the BBC he believes Mr Putin started a third world war with the invasion of Ukraine.

“I believe that Putin has already started it. The question is how much territory he will be able to seize and how to stop him… Russia wants to impose on the world a different way of life and change the lives people have chosen for themselves,” he told the broadcaster.