The six Britons, a Canadian and a Belgian confessed on Saudi television to carrying out bombings in Riyadh in 2000 and 2001 that killed a Briton and an American. They later withdrew their statements.
A Canadian official told Reuters that while the outlines of a deal to free the eight men had been in place for some time, the planned release had now hit a snag.
"Something has come up to delay the process. We don't know precisely what has caused that delay," the official said.
"Everybody expected it would be done sooner than this. This is a fairly complex deal for the Saudis to put together but we still have expectations this is going to happen and may happen soon," the official added.
Saudi officials say the bombings were the work of Western rival gangs involved in alcohol smuggling.
But analysts have linked the attacks to rising anti-Western sentiment in the kingdom.
Saudi lawyers for the accused said in May they had lodged an appeal for clemency, which had been favorably received by Saudi authorities.
The final decision on whether to grant clemency rests with Saudi King Fahd bin Abdel Aziz.
"Things take a long time in Saudi Arabia because this is a very sensitive issue that gets to the very center of the Saudi government," the Canadian official said.
"There is an arrangement in place that was done several months ago. We've had assurances from the summer of 2001 that there was going to be a political solution here," the official added.
In London, a Foreign Office spokesman declined to comment on efforts to free the men.
"We remain deeply concerned about the men's situation but we continue to work vigorously to resolve the cases and raise them regularly with the Saudi government," he said.