Around 500 British troops are being kept on standby for an emergency mission in Libya, a defence official said.
Troops of the Black Watch battalion of Scotland were secretly made ready on the orders of Prime Minister David Cameron, The Sun reported.
Around 200 of them have already moved to an air base in southwest Britain.
A warship - Type 23 frigate HMS Westminster - has also sailed for Libya.
Opponents of Libyan leader Muammar Gaddafi are demanding an end to his 42-year rule. Over 2,000 people have been killed in clashes in Libya that began Feb 14.
A defence ministry official said: "The whole region is extremely unstable at the moment, so it makes sense to have the Black Watch on immediate standby."
However, the most likely task for the Black Watch would be a humanitarian mission rather than a combat role.
On Friday, there were reports of several deaths in al-Zawiyah, 50 km west of Tripoli. Broadcaster Al Arabiya reported that 13 people were killed, while Al Jazeera said there were 50 dead and 300 wounded.