By Catarina Demony and Alistair Smout

LONDON (Reuters) -Britain has barred Angolan billionaire Isabel dos Santos and Ukrainian oligarch Dmytro Firtash and has frozen their UK assets, the government announced on Thursday, in what it said was part of a new crackdown on ‘dirty money’.

The measures were the first step in tightening Britain’s anti-corruption sanctions regime as promised in July’s election, the Labour government said.

“These unscrupulous individuals selfishly deprive their fellow citizens of much-needed funding for education, healthcare and infrastructure – for their own enrichment,” Foreign Secretary David Lammy said in a statement.

Dos Santos, whose father Jose Eduardo dos Santos ruled Angola for 38 years until 2017, is Africa’s first female billionaire and has faced corruption accusations in Angola and elsewhere for years. She denies the allegations and says she is the target of a long-running political vendetta.

Firtash is wanted by Ukrainian and U.S. authorities on suspicion of embezzling nearly $500 million involving Ukraine’s gas transit system. He says the charges are without legal foundation.

A Latvian businessman was the third main individual sanctioned, with Britain also announcing measures against five other individuals linked to the trio.

Lammy said the penalties were the start of a crackdown.

“I committed to taking on kleptocrats and the dirty money that empowers them when I became Foreign Secretary and these sanctions mark the first step in delivering this ambition,” he said.

Britain said dos Santos abused her position at Angolan state oil firm Sonangol and telecoms company Unitel to embezzle at least 350 million pounds ($440 million).

Dos Santos lost an appeal to overturn an order freezing up to 580 million pounds of her assets in September as part a lawsuit at London’s High Court brought by Unitel. Global police agency Interpol has issued a red notice for her.

Dos Santos told Reuters in a statement that the British sanctions were “incorrect and unjustified”.

“I was not given the opportunity to defend myself against these allegations,” she said. “I intend to appeal and I hope that the United Kingdom will give me the opportunity to present my evidence.”

Britain said Firtash had extracted “hundreds of millions of pounds from Ukraine through corruption”, and hidden tens of millions of pounds of ill-gotten gains in the UK property market alone.

Britain also sanctioned his wife Lada Firtash, who it said held UK assets on his behalf including the site of the old Brompton Road underground rail station in London.

Latvian businessman and politician Aivars Lembergs, who was put on a U.S. sanctions list in 2019 for alleged corruption, was also sanctioned, as was his daughter Liga Lemberga. The British government said Lembergs had “abused his political position to commit bribery and launder money.”

($1 = 0.7914 pound)

(Reporting by Catarina Demony, Alistair Smout and Muvija M in London; Additional reporting by Sergio Goncalves in Lisbon; Editing by Sarah Young, William Schomberg, Philippa Fletcher and Jonathan Oatis)