Pakistan's prime minister arrived in London on Wednesday for medical treatment over a previously unknown chronic heart condition, officials said.

Nawaz Sharif has been under tremendous pressure from opposition parties to step down since his family members were named in the mass leak of offshore account data from the Panama firm Mossack Fonseca.

Sharif's son Hussain Nawaz has owned offshore companies and one of the most expansive Mayfair London properties. One of his offshore accounts in the data is worth of 5.5 million British pounds, or about $7.8 million.

Pakistan's interior minister, Chaudhry Nisar Ali Khan, sought to assure the public Wednesday that Sharif will return in a few days.

The prime minister had an intervention on his heart some years ago that went "terribly wrong," Khan told reporters in Islamabad.

It was the first time that Sharif's heart condition was disclosed, though there were no details on the nature of the condition. Khan said that since the surgery, Sharif has had heart problems.

After the Panama leaks scandal erupted, Sharif insisted in a televised address to the nation that all of his family businesses were legal.

But Pakistani cricket-star-turned-politician Imran Khan has demanded an unbiased probe into the wealth the Sharif family had amassed. On Wednesday, he asked the government to set up a commission headed by a serving supreme court judge with experts on white collar crimes as members.

If the government fails to do this, Khan's party will march on Sharif's palatial farmhouses in the eastern city of Lahore he warned.

"They steal money and then they stash this money in foreign accounts," Khan alleged.

Khan, the interior minister, said the government was already working to set up the commission.