Pakistani Prime Minister Yousuf Raza Gilani says he will not write a letter to Swiss courts asking them to reopen graft cases against Pakistan's President Asif Ali Zardari.

Speaking to students Thursday, Mr. Gilani said writing the letter would be a violation of Pakistan's constitution, amounting to treason, a crime that carries the death penalty.

The prime minister said that if he does not write the letter, he faces a six-month jail term for contempt.

Last week, Pakistan's Supreme Court asked Mr. Gilani to personally write Swiss authorities to reopen an old corruption case against the president.

In February, the court charged Mr. Gilani with contempt for defying its order that he reopen the case. That made the prime minister Pakistan's first chief executive to be indicted while in office.

Mr. Gilani pleaded not guilty and vowed to contest the charges. If convicted, he could lose his job as well as face time in prison.

In December 2009, the Supreme Court struck down a controversial amnesty law passed under the former president Pervez Musharraf that nullified corruption charges against thousands of politicians, including the president, dating back to the 1990s. The court ordered Mr. Gilani's coalition government to revive all the cases within Pakistan and to write a letter to Swiss authorities to do the same.

Prosecutors accuse Mr. Zardari, former Prime Minister Benazir Bhutto and others of laundering millions of dollars through Swiss bank accounts. Ms. Bhutto, President Zardari's wife, was assassinated in 2007.

Mr. Zardari has denied the allegations and Prime Minister Gilani has refused to cooperate, arguing that the president has legal immunity while in office.