Pakistani judges on Friday agreed to allow star witness in a major scandal threatening President Asif Ali Zardari to testify abroad, resurrecting an investigation that appeared to come unstuck.

American businessman Mansoor Ijaz alone implicated Zardari in a controversial memo seeking US help to rein in Pakistan's powerful military last May, but has refused to travel to Pakistan, citing fears for his safety.

His testimony is considered key to any case against the president, who faces frenzied speculation that he could be forced out of office over the scandal that dates back to chaotic days after US troops killed Osama bin Laden.

"The commission has decided to record the statement of Mansoor Ijaz through video link on February 22 at 2:00 pm (0900 GMT)," his lawyer Akram Sheikh told AFP. Ijaz will give the statement at Pakistan's high commission in London.

A commission member will also travel to London to collect evidence, including BlackBerries, other devices and forensic material.

"It is an important development in this case. The court has accepted my request. We want the people should know what is the truth," Sheikh said.

The judicial investigation appeared at risk of collapsing on January 24, when investigators ruled out travelling abroad to hear the testimony.

Although the Supreme Court subsequently extended the commission's mandate by another two months, Ijaz continued to refuse to travel to Pakistan.

Zardari's ambassador to Washington, Husain Haqqani, was forced to resign over the scandal but he flatly denies writing the memo.

The Supreme Court ordered an investigation into the scandal following advice from the head of Pakistan's intelligence agency.

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

Writing in the Financial Times on October 10, Ijaz alleged that a senior, unnamed Pakistani diplomat telephoned him asking for help because Zardari needed to communicate an urgent message to the Americans.