Haqqani was forced to resign as ambassador to Washington over claims that he was involved in drafting a secret memo trying to enlist US help to curb the power of the military. He denies the accusations.

The Supreme Court restricted his travel and ordered judges to investigate who was behind the memo, heightening frenzied speculation that President Asif Ali Zardari could be forced out of office.

"He is allowed to proceed out of country," chief Justice Iftikhar Muhammad Chaudhry said in his order, after lawyer Asma Jehangir guaranteed her client would appear before judges at four days' notice if required.

Last week, the investigation appeared to come unstuck when judges ruled out travelling abroad to hear the testimony of American businessman Mansoor Ijaz, who alone had implicated Zardari in the document.

Although Ijaz has been given another chance to appear before a judicial commission on February 9 and the Supreme Court on Monday extended its mandate by an extra two months, he has refused to visit Pakistan over security fears.

"I am glad that the Supreme Court has restored my right to travel, which had been rescinded without any charges being filed against me. I will join my family in the US after discussions with the leaders of the (main ruling) Pakistan People's Party," Haqqani told AFP in an email.

The memo was delivered on May 10 to Admiral Mike Mullen, then chairman of the US Joint Chiefs of Staff, and allegedly aimed to forestall a feared military coup after American troops killed Osama bin Laden in Pakistan.

The Supreme Court ordered an investigation into the scandal following advice from the head of Pakistan's intelligence agency, who said Ijaz had evidence worth investigating.