Sources in Mirza's family were quoted on the website of the Dawn newspaper as saying that he would meet Scotland Yard officials in the next few days.
Mirza, who has settled with his family in Dubai, flew to London after he was contacted by British officials, the report said.
British police are investigating allegations that Hussain was involved in money laundering and inciting violence in Karachi, Pakistan's largest city. Mirza, who was once close to President Asif Ali Zardari, has served as Home Minister of Sindh province.
Scotland Yard officials have reportedly requested Mirza to produce evidence he had offered to present against the Muttahida Qaumi Movement and Hussain in 2011.
The ongoing investigation against Hussain, confirmed by a BBC news programme, has caused widespread unrest and tension in Karachi. The BBC reported that 400,000 pounds were found during raids on Hussain's home and a MQM office in London.
The MQM has been a dominant force in Karachi since the 1990s. In the recent general election, the MQM won a majority of parliamentary and provincial assembly seats in Karachi.