Altaf Hussain, chief of Pakistani political party Muttahida Qaumi Movement (MQM), is being treated at a London hospital for the past one week, a statement said on Monday.
The party statement, however, did not reveal details about his illness, Geo News reported.
The MQM, which was a part of the federal coalition led by the Pakistan Peoples Party of President Asif Ali Zardari, quit earlier this year due to differences on a range of issues ranging from power sharing to the war on terror.
Hussain who lives in exile in London, regularly speaks to his followers in Karachi on the telephone, attracting tens of thousands of people. He fled Pakistan in 1992 after a military operation in Karachi, requested political asylum in Britain and has now become a citizen of the country.
Senior MQM leader Imran Farooq, a close aide of Hussain, was killed in an ambush in London in September 2010.