President Joe Biden has faced criticism for supplying the munitions, which are banned by many allies because of their track record of killing and maiming civilians.

The prime minister said the UK was one of 123 countries that signed a convention banning their use, and would continue focusing on supplying tanks and long-range weapons to help the fight against Russia.

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He added: "We will continue to do our part to support Ukraine against Russia's illegal and unprovoked invasion, but we've done that by providing heavy battle tanks and most recently long-range weapons, and hopefully all countries can continue to support Ukraine.

"Russia's act of barbarism is causing untold suffering to millions of people.

"It's right that we collectively stand up to it."

Mr Sunak will meet Mr Biden in London on Monday ahead of a NATO summit.

Why cluster bombs decision is controversial

Cluster bombs detonate in the air and release "bomblets" that scatter over a large area.

Opponents say they kill indiscriminately and that some of the smaller munitions can fail to detonate, posing a long-term risk to civilians.

Mr Biden has called it a "difficult decision" but said he had to act as "the Ukrainians are running out of ammunition".

The US says Kyiv has provided assurances it will not use cluster bombs in urban areas, but some NATO allies are likely to be uneasy over their supply.

The Convention on Cluster Munitions prohibits their use or stockpiling because of their indiscriminate effect on civilian populations, though the US, Ukraine, and Russia are not signatories.

Both Moscow and Kyiv have used cluster munitions so far in the war.

Ukrainian President Volodymyr Zelenskyy thanked the US for the "timely, broad and much-needed defence aid package" that will "bring Ukraine closer to victory over the enemy, and democracy to victory over dictatorship".

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Biden's made 'wrong call'

Commons Defence Committee chairman Tobias Ellwood urged the US to "reconsider".

The Tory MP tweeted: "This is the wrong call and will alienate international good will. Their use leaves deadly unexploded ordnance over the battlefield, killing & injuring civilians ."

But Dr Yuriy Sak, adviser to Ukraine's defence minister, told Sky News: "It is an important move. It is a timely move.

"Russia has been using cluster munitions from day one of this aggressive, unprovoked war against Ukraine. They've been using cluster munitions against civilian cities.

"Now, we need these types of weapons as well as other types of weapons, in order for us to achieve one single sole purpose, which is to liberate our land."

He added: "We will be using these weapons very responsibly.

"Our minister of defence has given written assurances that certain principles will be applied.

"We have a right to self-defence… but in exercising that right, we will always commit ourselves to abiding the international norms of war and humanitarian laws.

"We are dealing with an enemy who is killing our civilians every day."

'The only target will be the enemy'

Insisting the weapons would not be used in urban areas, Dr Sak said: "They will be used only on the battlefield to allow the Ukrainian army to break through the defence lines of the enemy.

"The only target will be the enemy."

Germany, which is signed up to the cluster bomb ban, has expressed understanding for the American position.

A spokesman for the Berlin government said: "We're certain that our US friends didn't take the decision about supplying such ammunition lightly.

"We need to remember once again that Russia has already used cluster ammunition at a large scale in its illegal war of aggression against Ukraine."

But Spain, also a signatory to the convention, said it opposed the decision.

Spanish defence minister Margarita Robles said: "Spain, based on the firm commitment it has with Ukraine,
also has a firm commitment that certain weapons and bombs cannot be delivered under any circumstances."