David Cameron has taken to Twitter to condemn the "sickening" terrorist attack that killed 22 people in Tunisia today.
The Prime Minister said he was "appalled" by the attack and pledged British support to the North African country.
Gunmen in military uniform stormed the Bardo Museum today in the capital Tunis, picking off tourists who were visiting its world-renowned collection of Roman mosaics.
Mr Cameron wrote: "Appalled by sickening terrorist attack in Tunis - my thoughts are with those affected. UK stands ready to support Tunisia."
The Foreign Office said it was "urgently" looking into the attack amid reports Britons may be among the dead.
However, the Tunisian prime minister said the victims are Polish, Spanish, German, Italian and Tunisian, according to the BBC. Twenty of the 22 dead were foreign tourists.
Eyewitnesses told how the killers picked off visitors one by one, while others hid behind pillars or fled in terror to escape.
Security forces reportedly shot dead two gunmen and were searching for accomplices.
A Foreign Office spokesman said: "We are urgently looking into the serious events in Tunisia and are working with the Tunisian authorities as well as with contacts in the tourist industry to establish if any British nationals have been affected.
"Our travel advice is that we are urging people to exercise caution and follow the instructions of the local authorities."
British tour operators confirmed that thousands of customers are believed to be in the country, but "very few" are in the capital where the attack was carried out.
A spokesman for the Association of British Travel Agents (Abta) said: "We are currently liaising with our members and the Foreign Office to assess the impact and understand if any British nationals have been caught up in the attack on the Bardo Museum in Tunis.
"Tunisia attracts over 400,000 UK visitors each year, with the vast majority travelling to resorts on the coast.
"Abta members have reported that they have currently approximately 6,500 customers in Tunisia, though very few of these are staying in Tunis. As a precaution many Abta tour operators have cancelled local excursions to Tunis due to depart over the next few days."
The National Bardo Museum is near the national parliament building, which was evacuated after the shooting.
The museum is a leading tourist attraction that chronicles Tunisia's history and houses one of the world's largest collections of Roman mosaics.
