David Cameron considered ordering British special forces to board and impound a Russian ship suspected of carrying arms to Syria, it emerged on Wednesday.
Cobra, the Government's emergency security committee, met several times as the MV Alaed approached British waters.
With the United States placing pressure on Britain to halt the vessel, the Prime Minister was briefed regularly on the situation.
It is understood that he was presented with several options including a military seizure of the ship.
Avoiding a confrontation that could have damaged already strained ties with Russia, the Government instead took action to ensure that the Alaed's insurance cover was withdrawn.
The ship, which Western officials said was carrying a military cargo including Hind-D Mi-25 helicopter gunships and anti-aircraft defence systems, changed course about 50 miles off the north coast of Scotland. It is now showing that its next port of call is Murmansk, according to the UK National Maritime Information Centre.
The ship's owners, the Russian operator Femco, denied yesterday that it was ferrying arms to Syria.
"Reports about illegal actions of the Alaed's owner are inaccurate," it said in a statement. "The vessel is currently making a regular commercial voyage, in full accordance with international norms and rules."
In Syria, the Red Cross said it would try to evacuate hundreds of civilians trapped in and around the city of Homs.
At least 60 people - more than a third of them government troops - were killed in the country yesterday, according to the Syrian Observatory for Human Rights.
Bashar Jaafari, the Syrian ambassador to the United Nations, yesterday compared the bloody uprising to last summer's riots in Britain.
About 15,000 people have been killed in Syria compared with five deaths during the riots that swept England for four days in August.
Sir Mark Lyall Grant, the British ambassador to the UN, called the comments "utterly grotesque".
