Pakistan on Tuesday said it wrote to British authorities seeking the return of former prime minister Nawaz Sharif, who flew to London for medical treatment in 2019 after a court granted him indefinite bail on medical grounds. Sharif, a former three times PM, was ousted over corruption allegations in 2017 and was serving a seven years prison term before the Lahore High Court granted him an indefinite bail on medical grounds, after which he flew to Britain for medical treatment. Pakistan cannot seek Sharif's extradition because it has no extradition treaty with Britain, the foreign office said. "A letter to bring back Nawaz Sharif's has been written through foreign office... This letter was a legal formality to bring him back to the country, which has been sent", advisor to prime minister Imran Khan on information and broadcasting Firdous Ashiq Awan said at a press conference. According to the bail terms, Sharif was required to send his medical reports to a medical board set up by the court, which would then determine if his bail could be extended -- but Sharif has sent no such information, Awan said. "The medical board demands his medical report, but he sends letters and certificates, there is a difference in medical report and letters", she said. Sharif's doctor last year claimed that he suffered a minor heart attack and was "fighting for his life" in prison. Sharif's younger brother Shehbaz Sharif, the leader of the opposition, said in a statement that the Pakistani government "has no legal authority" to write such a letter to Britain. "It is an absolute immoral and irrational act, the government's haste reveals its criminal and malicious intentions," the statement read. Former PM Sharif, 70, known as the "Lion of Punjab", is a political survivor who has repeatedly roared back to the country's top office, underscoring the unpredictable nature of Pakistani politics. A hugely wealthy steel tycoon from Punjab, Pakistan's wealthiest province, Sharif earned a reputation for combativeness during his first two terms as prime minister, from 1990 to 1993 -- when he was sacked, also on corruption charges -- and from 1997 to 1999, when he was deposed by the powerful military. His first two terms were marked by a strong economy and infrastructure growth. However Sharif inherited a country with sagging finances and a stifling energy crisis when he was elected for the third time in 2013. Sharif blamed the security establishment for again targeting him in 2017 when the Supreme Court disqualified him from politics for life over graft allegations, which he denies. He later received a seven-year jail sentence. Prime Minister Imran Khan, who defeated Sharif's party in 2018 general election, has since launched a high-profile and controversial anti-corruption drive that has targeted several leaders of Sharif's party.