MI5 made "substantial contributions" to an investigation which has seen four people - including two members of the Russian security services - charged over one of the largest data breaches in US history.
The US Justice Department announced on Wednesday that charges have been brought in relation to the 2014 attack which targeted around 500 million Yahoo accounts.
Acting Assistant Attorney General Mary McCord said: "We're here for one of the largest data breaches in US history.
"Today we are announcing the indictment of four individuals responsible for the 2014 hacking... of Yahoo, the theft of information about at least 500 million Yahoo accounts, and use of that information to obtain the contents of accounts at Yahoo and other providers."
FBI executive assistant director Paul Abbate said: "We are extremely grateful as well to our international partners for their assistance and support leading up to these criminal charges today.
"Those partners include Canada's Royal Canadian Mounted Police and, as mentioned, the Toronto police service and their fugitive squad.
"As well, the United Kingdom's MI5 made substantial contributions to the advancement of this investigation also."