The MV Alaed has had to turn back after its insurers withdrew cover when it emerged that the vessel was carrying attack helicopters and arms from Russia to Syria in contravention of an EU embargo.
The ship was about 50 miles (80km) off the north coast of Scotland when it was forced to stop when insurers the Standard Club in London scrapped its insurance cover, the Telegraph reported.
The ship was not allowed to sail until its owner could secure new cover.
Reports said the ship had stopped off the Western Isles for some time but foreign secretary William Hague said it was on its way back to Russia, the BBC reported.
The move came after other reports of Russia sending shipments of weapons to Syria in defiance of a European Union arms embargo emerged.
The Foreign Office said: "We are aware of a ship carrying a consignment of refurbished Russian-made attack helicopters to Syria.
"The foreign secretary made clear to Russian foreign minister [Sergey] Lavrov when they met on 14 June that all defence shipments to Syria must stop.
"We are working closely with international partners to ensure that we are doing all we can to stop the Syrian regime's ability to slaughter civilians being reinforced through assistance from other countries."
The Standard Club said insurance was withdrawn because the owner had broken the club's internal rules. It insisted Whitehall was not behind the move.
"We were informed on Friday evening that the ship might be carrying weapons, in particular attack helicopters, missiles and non-specific munitions, and we are making inquiries to establish what their side of the story is," it said.
"There are exclusion clauses in our cover and, for anyone involved in improper or unlawful trade, we can cancel cover."
Femco, a Russian cargo line that manages the ship, told AFP: "We were made aware of the allegations that the Alaed was carrying munitions destined for Syria.
"We have already informed the ship owner that their insurance cover ceased automatically in view of the nature of the voyage."