LONDON (AFP) - London police have seized a trove of cryptocurrency worth £114 million (S$212 million) that criminals were planning to launder, Britain's largest-ever seizure of the digital assets.

The Metropolitan Police on Friday said they arrested a 39-year-old woman on suspicion of money laundering after detectives seized the digital currency, which they believe was being used to transfer proceeds of crime.

The police statements did not say what type of cryptocurrency was involved.

"As technology and online platforms develop, some are moving to more sophisticated methods of laundering their profits", said deputy assistant commissioner Graham McNulty.

"But we have highly trained officers and specialist units working day and night to remain one step ahead," McNulty added.

It was "the largest cryptocurrency seizure in the UK and believed to be one of the largest globally," the police force said.

Cryptocurrencies are hard to trace, in turn making them difficult to regulate.

Bitcoin, the world's most popular virtual unit, was created in 2008 as an alternative to traditional currencies.

Cryptocurrencies are minted by solving puzzles using powerful computers that consume enormous amounts of electricity.

Bitcoin fell below US$30,000 on Tuesday for the first time since January, down from its high of US$64,870 reached in April.