The Russian oligarch Roman Abramovich has stepped up his row with the British government over the £2.5bn proceeds of his sale of Chelsea FC, insisting that the money is his to allocate despite the international sanctions imposed on his assets.

The UK and EU imposed sanctions on Abramovich in 2022, freezing his assets in response to Moscow’s full-scale invasion of Ukraine, citing his ties to Vladimir Putin’s regime.

The move triggered a rushed sale of the Premier League club to a consortium led by the US businessman Todd Boehly.

However, the £2.5bn raised by the sale has remained locked in a UK bank account since then because of a dispute about how it should be used.

Britain wants the money to be ringfenced for use only in Ukraine, in line with a wider European push for Moscow to foot the bill for the devastation caused by its invasion.

However, Abramovich has indicated that he wants more flexibility over how the money would be spent.

When he announced his intention to sell the club in March 2022, he said the proceeds would be used “for the benefit of all victims of the war in Ukraine”, leaving open the possibility that money could be diverted to Russian recipients.

On Monday, Abramovich’s lawyers at Kobre & Kim said the cash was still “wholly owned” by Fordstam Ltd, the vehicle through which the billionaire funded Chelsea, channelling cash from controversial oil deals through a maze of offshore companies.

In the letter, sent ahead of a 17 March deadline set by the UK government, the lawyers accused ministers of making “politically charged and highly publicised statements” about the oligarch.

They said Abramovich remained fully committed to using the money for charitable purposes, and the government’s restrictions on how it can be spent were to blame for the delay.

“The UK government appears to be treating this proposed donation as a form of punitive measure against Mr Abramovich,” the lawyers said in the letter, seen by Reuters.

In response to the letter, Britain’s foreign secretary, Yvette Cooper, said: “This money was promised to Ukraine over three years ago. It is time Roman Abramovich does the right thing but if he won’t we will act”.

The British government warned Abramovich last year he must release the cash, or he could be taken to court.

Keir Starmer said in December “the clock is ticking” over finding a resolution.

However, Abramovich’s lawyers said if the government opened formal confiscation proceedings it would be contested in court.

“The proposal to donate these proceeds was initiated by Mr Abramovich prior to the imposition of sanctions, and he remains fully committed to ensuring that the funds are used for charitable purposes,” the letter said.