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Cdr Kevin Southworth - who leads public protection at the Metropolitan Police - told the BBC officers were still working their way through the 23 potential new cases and that some of the people were "definitely not identical" to those featured in Zou's seized secret footage or from the charge cases for far.

"We want to avoid situations where victims feel like they're maybe not being taken seriously, or heaven forbid, being disbelieved," says Cdr Southworth.

The officers were assisted by the Chinese authorities, who have been working collaboratively with the Met and are "very supportive", says Cdr Southworth.

Met counter-terrorism chief Cdr Murphy said the case was an "extremely sophisticated" operation that "posed a threat to national security and individuals".

The methods they used were the sort of thing you would "expect to see in a spy novel", said the Metropolitan Police's Cdr Dominic Murphy.