ISLAMABAD -- Pakistani opposition parties have urged the country's Prime Minister Nawaz Sharif to step down after a Supreme Court probe into corruption allegations against his family members found "significant disparity" between their declared wealth and known sources of income.

One of the key pieces of evidence to emerge from the court-appointed Judicial Investigation Team's (JIT) work is a collection of documents submitted by Sharif's daughter Maryam.

The documents -- which Maryam Sharif presented to the JIT as having been created in 2006 -- were written in Microsoft's default Calibri font. That font was not made widely available to the public by Microsoft until the following year.

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The JIT discovered the discrepancy, since touted by many in Pakistan as evidence of fraudulent evidence, after sending the documents abroad for forensic analysis.

The daughter of Pakistani Prime Minister Nawaz Sharif, Maryam Nawaz, arrives to appear before an anti-corruption commission at the Federal Judicial Academy in Islamabad, July 5, 2017. Getty

Sharif has been under pressure since documents leaked in 2016 from a Panama-based law firm disclosed that his family had offshore accounts. He's been accused of hiding his family's wealth by moving around money in those accounts in illegal transactions.

Babar Awan, a leader of cricket-star-turned-politician Imran Khan's party, said on Tuesday that Sharif's only option is to quit. Sirajul Haq, the leader of opposition Jamaat-e-Islami party, also demanded Sharif's resignation.

Investigators in the probe suggested on Monday that courts pursue action based on a 1999 accountability law intended to help eliminate corruption.

But the final decision rests with the Supreme Court, which will take up the case next Monday. The JIT is expected to present its findings formally to the court at that time, and the Court could rule on the corruption allegations against Sharif as soon as next week.

The family of Sharif, prominent industrialists from Lahore, has owned steel mills, including sites in Saudi Arabia, where he was sent into exile by then-President Gen. Pervez Musharraf in 2000.

Sharif entered politics in the 1980s, and won his second term as Prime Minister in a 2013 election, in large part by convincing lower and middle classes that he could revitalize the country's economy.