Pakistan‘s supreme court is to investigate the scandal surrounding an unsigned memo – reportedly sent by a close ally of the president, Asif Ali Zardari, to the US after Osama Bin Laden‘s killing – asking for Washington’s help in staving off a military coup.

Husain Haqqani, the Pakistani ambassador to the US, was named in the Financial Times by businessman Mansoor Ijaz as the author of the note. Haqqani denied involvement but resigned over the ensuing controversy, which revived historic tensions between the government and the military.

The court’s decision on the “memogate” scandal is bound to heap pressure on the weak civilian government and embolden Pakistan’s military, which has held power for more than half Pakistan’s 64 years and whose help Washington needs to quell militants fuelling violence in neighbouring Afghanistan.

There is widespread speculation the court’s judicial commission, which has been told to report in four weeks, could spell the end of Zardari’s presidency if a link to the memo is established.

“I think that this is one of the darkest days in history for the judiciary,” Haqqani’s lawyer, Asma Jahangir, told reporters. “I said in the supreme court too that this is a very disappointing judgment. This is a judgment that places national security above fundamental rights.”

The court has also directed the government to contact Research in Motion to obtain verification of Ijaz’s claims of BlackBerry conversations with Haqqani.

The government has become increasingly unpopular since Zardari took office in 2008, failing to tackle myriad problems from crippling power cuts to suicide bombings and a struggling economy. Speculation had swirled of an imminent coup, but all sides have since dismissed the rumours as baseless. Analysts say the military has plenty of other ways to pressure Zardari to step down.

Human Rights Watch said justice must be seen to be done in the investigation. “In a sense, ‘memogate’ is a litmus test for all actors – particularly the judiciary and the army,” it said in a statement. “It remains to be seen whether the rule of law or the law of the jungle prevails in Pakistan.”

Although his position is largely ceremonial, Zardari wields considerable influence as leader of the ruling party and his forced departure would be a humiliation for the civilian leadership and could throw the country into turmoil.