Image copyright Reuters Image caption Nawaz Sharif at a news conference in London in 2018

Pakistan's former prime minister Nawaz Sharif, who is serving a prison sentence for corruption, has left the country to receive medical treatment in London.

Sharif, a three-time prime minister, has an immune system disorder and other health problems.

The Lahore High Court lifted a travel ban on Saturday and the government rubber-stamped the decision on Monday.

Imran Khan's government had been reluctant to release Sharif.

It wanted the politician - who has so far served less than 12 months of a seven year prison sentence - to sign an indemnity bond worth $44m (£34m) before allowing him to go abroad.

However, the court allowed him to travel without signing the bond.

How ill is Nawaz Sharif?

Sharif's health and the travel ban have dominated news in Pakistan for nearly a month.

Last month, his blood platelet count dropped to a critical 16,000, compared to around 150,000 for a healthy person, a doctor told BBC Urdu's Umar Nangiana in Lahore.

The 69-year-old is also on drugs for a cardiac problem, and suffers from diabetes and kidney issues.

Image copyright AFP Image caption Supporters pray outside a hospital where Sharif was being treated in October

Both official and private medical boards have recommended he is treated abroad, as Pakistan does not have adequate facilities.

Sharif will travel via Doha in a "high-end air ambulance" with a fully-staffed intensive care unit and operating theatre, his personal doctor said.

Why is he serving a jail sentence?

Sharif was prime minister from 1990 to 1993, then again from 1997 to 1999, and finally from 2013 to 2017. He is also one of the country's richest industrialists.

In 2018 - following the Panama Papers leak - he was found guilty of corruption, relating to his family's ownership of upmarket London apartments.

He was given a 10-year prison sentence, but was released two months later when the court suspended the sentences, pending a final judgement.

But in December 2018 he was jailed for corruption again, this time for seven years, in relation to his family's ownership of steel mills in Saudi Arabia.

He denies wrongdoing and accuses the military of conspiring to end his political career.

When will he come back?

Sharif has agreed to return within four weeks, or when doctors say he is fit to return.

His brother Shahbaz - who is travelling with Sharif - signed a court document that said: "If at any stage, the federal government has credible information that Nawaz Sharif is living abroad despite his fitness to travel, a representative from Pakistan's High Commission would have a right to meet with his physician(s) to verify or confirm about his health."