Pakistani Prime Minister Yousuf Raza Gilani has no option but to write to the Swiss authorities to reopen corruption cases against President Asif Ali Zardari, opposition leader Nawaz Sharif has said.

Gilani has refused to write to the Swiss authorities, as directed by the Supreme Court, arguing that Zardari enjoys immunity under the constitution.

Sharif, chief of the opposition Pakistan Muslim League-Nawaz (PML-N), said the prime minister "should think seriously" about writing the letter, and that the PML-N would stand by the Supreme Court if he did not do so, Geo News reported.

Accused of graft, Zardari had been granted amnesty under the National Reconciliation Ordinance (NRO) in 2007 by then president Pervez Musharraf to facilitate his return and, primarily, that of his wife, former prime minister Benazir Bhutto from self-exile.

The NRO that granted immunity to politicians and bureaucrats in corruption cases was struck down as void in 2009.

The apex court warned the government of action if its ruling on the NRO was not implemented by Jan 10, 2012. It further ordered the government to write a letter to the Swiss authorities to reopen the graft cases.

On Jan 16, the court issued Gilani a contempt notice for not acting against Zardari.

Gilani will now appear before the Supreme Court Feb 13 to be formally charged with failing to reopen the graft cases against Zardari. If convicted, he faces six months in prison and risks losing his parliamentary seat. He may also be slapped with a five-year ban from holding any public office.