Pakistan Tehreek-I-Insaf chairman Imran Khan has admitted forming an offshore company to buy a flat in London over two decades ago to evade British taxes, drawing sharp reaction from his political opponents who criticised him for repeatedly denying possession of any such firm.

Khan, in a dramatic admission, said that he formed an offshore company to buy a London flat in 1983 to evade British taxes, a day after his party officially announced that Imran did not own any such company.

"I was already paying 35 per cent tax on my income there, so to evade further taxes, I bought the flat through an offshore firm, which was my right as I was not a British citizen," Imran said at London's Heathrow Airport yesterday.

The admission came in the midst of his virulent campaign against Prime Minister Nawaz Sharif to resign after his two sons and a daughter were named in Panama Paper leaks as having offshore companies.

Panama Papers, a massive leak of 11.5 million tax documents that reportedly exposed the secret offshore dealings of around 140 political figures globally, named three of Sharif's four children -- Maryam, Hasan and Hussain -- listing them as owners of offshore companies.

Khan's party spokesperson Naeemul Haq admitted that his party chief had formed a 'legal' offshore company through his earnings from cricket, which owned his London flat.

Defence Minister Khawaja Asif criticised Khan for repeatedly denying the possession of any offshore firm.

Moreover, Sharif's daughter Maryam, through her Twitter account, termed Khan the "pioneer" of offshore companies.

Pakistan's information minister Pervaiz Rashid demanded Khan to quit politics for demanding probe against those having offshore companies.

Rashid termed Khan as the "godfather" of those having offshore companies as he was among the first Pakistanis who had this idea of "dodging" tax authorities.

"He has lost moral authority to be in politics as now it is established that he was hiding facts," the minister said.

He also said that at least six senior member of Khan's PTI own such companies which the highest number of people from a Pakistani political party.

"By his own definition of offshore entities, Khan and his party members are involved in white collar crime," said Rashid.