By Susannah Price BBC correspondent in Islamabad

Political parties in Pakistan have condemned a decree which in effect bans two former prime ministers, Nawaz Sharif and Benazir Bhutto, from returning to power.

This is part of a series of laws aimed at victimising political parties and political leaders

Pakistan Muslim League

The decree, issued by President General Pervez Musharraf on Saturday, prevents people who have held the office of prime minister twice in the past from being given a third term.

The section of the Pakistan Muslim League led by Mr Sharif said it was aimed at ousting the two big leaders from politics, and that they might challenge it in court.

A spokesman for Benazir Bhutto's Pakistan People's Party called it unconstitutional.

President Musharraf has repeatedly said he will stop the two former prime ministers from regaining any political power.

'Good governance'

There is widespread speculation about if, and when, Benazir Bhutto, who left Pakistan after being dismissed, would return, and the level of support she would now receive.

Bhutto faces arrest if she returns to Pakistan

Nawaz Sharif was ousted by General Musharraf in a coup in October 1999.

He went into exile in Saudi Arabia in a deal in which he pledged to take no further part in politics.

General Musharraf said the decision was taken to establish genuine and sustainable democracy, and ensure good governance.

He has pledged to hold elections in October, in line with a Supreme Court ruling.

Presidential power

However, he will remain as president for a further five years, after winning a controversial referendum.

Last week, he announced proposed constitutional amendments that

• would give him the power to dismiss the cabinet and parliament

• shorten some of the elected terms

• and bar criminals and those without degrees from standing in the elections.

Critics say that, despite the promise of polls, President Musharraf is trying to ensure that he will retain power over any future parliament and prime minister.