Sir Keir Starmer has warned Roman Abramovich that "the clock is ticking" for him to donate £2.5bn from the sale of Chelsea to Ukraine.
The Prime Minister told MPs on Wednesday that ministers have issued a licence allowing the proceeds from the sale of Chelsea to be transferred to a new foundation for humanitarian causes in Ukraine.
Speaking in the House of Commons, he said: "My message to Abramovich is this: the clock is ticking.
"Honour the commitment you made and pay up now, and if you don't, we are prepared to go to court so every penny reaches those whose lives have been torn apart by Putin's illegal war."
Abramovich sold the club in May 2022 after he was sanctioned following Vladimir Putin's invasion earlier that year.
The oligarch had pledged to donate the proceeds from the sale to the people of Ukraine, but he has so far failed to reach an agreement with the government on a way forward and the funds remain frozen.
Wednesday's decision to grant a licence for the transfer is an effort to force Abramovich to fulfil his promise before the government resorts to legal action.
The government previously raised the possibility of legal action in June, when Chancellor Rachel Reeves and then-foreign secretary David Lammy said they were "frustrated" by the failure to reach an agreement with Abramovich.
On Wednesday, Reeves said it was "unacceptable" that the money remained frozen in a UK bank and said ministers were "prepared to do what is necessary" to ensure the funds reach Ukraine.
The Treasury said ministers would consider any proposal from Abramovich to voluntarily transfer the money to Ukraine.
Under the new licence, the proceeds must go to humanitarian causes in Ukraine, while any future gains can be spent more broadly on victims of conflict around the world.
Your Site News' chief correspondent Kaveh Solhekol:
"There has been a stand-off between the UK government and Abramovich because the government wants the money to go to Ukraine, whereas Abramovich wants the money to go to "all victims" of the war, which would see some of it potentially going to Russia.
"Abramovich was sanctioned at the time of the sale and the sale only went ahead because he was issued with a special license which allowed the sale, providing he could prove he would not personally benefit.
"The money still legally belongs to Abramovich, but he cannot access it.
"What we've seen the government say today is 'We've run out of patience. This has gone on for too long. We are going to make you release these funds so they can go to the victims of the war in Ukraine, not Ukraine and Russia'.
"Abramovich never wanted to sell Chelsea at all. He didn't need the money. He was very popular with Chelsea supporters and always came across as somebody who loved the club.
"He was forced to sell Chelsea under the terms of a special licence, which meant the money had to go to victims of the war in Ukraine. Politically, that is difficult for him.
"I'm not sure how easy it is going to be for the UK government to force him to release this money, but the prime minister has said the clock is ticking. It is ridiculous this money has just been sat frozen in a bank account. The government are getting tough now and saying enough is enough - but I don't think it's going to be simple."
