A preliminary hearing into the deaths of two unarmed IRA men shot dead by the SAS almost 20 years ago is due to open today.

Martin McCaughey (23) and Dessie Grew (37) were shot dead by SAS soldiers at a farm building near Loughgall, Co Armagh in October 1990.

The preliminary hearing is scheduled to take place at Mays Chambers in Belfast.

The families of the two republicans have mounted a series of legal challenges over the RUC investigation into the killings and the failure to hand over intelligence documents to allow inquests to be held into the deaths.

It remains the longest outstanding inquest in Northern Ireland’s legal history.

The two men were shot close to outbuildings at Lislasley where three AK47s were found nearby, although the two men were unarmed.

Postmortem examinations revealed that Grew had been shot 48 times and McCaughey 12 times.

The shootings became part of a series of unarmed security force killings known as ‘shoot to kill’ after it emerged that the shed had been under surveillance and that police had prior intelligence that the two IRA men were due to visit.

In May 2001 the European Court of Human Rights (ECHR) ordered the British government to pay £10,000 each to the families of 10 IRA men shot dead by the security forces after the court ruled that police had not properly investigated their deaths.

In November 2007 the ECHR ruled that the RUC had also failed to properly investigate allegations of security force collusion in the killing of eight Co Armagh men.

One month later the House of Lords ruled that all police intelligence files relating to the Grew and McCaughey killings should be disclosed to the coroner to allow full inquests to take place.

Last month it was revealed that the two men’s families were taking their case to Europe claiming there was no proper police investigation into their killings.

Martin McCaughey’s mother Brigid and Dessie Grew’s father Patrick have launched a legal challenge at the ECHR.

Speaking last month, a spokesperson for the families solicitors Fearghal Shiels of Madden & Finucane said the relatives believed that the state had “clearly breached its legal obligations to conduct an effective official investigation into the deaths”.

“The RUC officers who investigated the killings lacked the requisite degree of independence from the undercover soldiers involved in the shooting,” the spokesperson said.

“No attempt was made to seriously challenge the excessive force used, involving the firing of at least 72 rounds, and in circumstances where one of the men was shot twice on the ground as he was dying or already dead.

“There was clearly no meaningful attempt made by the RUC to explore the credibility of the accounts provided, failing even to re-interview the soldiers in light of significant discrepancies in their accounts.”