Britain's Foreign Office (FCO) advised against all but essential travel to Tunisia today, telling Britons to leave the North African country and warning that further terrorist attacks were "highly likely" there.

Thirty Britons were killed by an Islamist gunman in Tunisia on June 26, the worst loss of British lives in such an incident since bombings in London in July 2005.

The FCO said it was working with tour operators including Thomas Cook and TUI Travel , to bring holidaymakers back to the UK.

British authorities did not believe that the security measures put in place in Tunisia were sufficient to keep holidaymakers safe, Foreign Minister Philip Hammond said in a statement.

"Since the attack in Sousse the intelligence and threat picture has developed considerably, leading us to the view that a further terrorist attack is highly likely," he said.

Britain had already said Islamist militants could launch further attacks on tourist resorts.

Thomas Cook said in a statement on Thursday it was working to bring home 2,000 British and Irish guests currently at its resorts in Tunisia on 10 flights scheduled for the weekend.

Both Thomas Cook and TUI, which operates Thomson and First Choice holidays, said they had cancelled all future bookings to the country up until the end of October. TUI said it did not currently have any customers in the country.

A total of 38 people were killed when Saif Rezgui opened fire on holidaymakers at the resort of Sousse two weeks ago. In March, two gunmen killed 24 people dead, 21 of them foreign tourists, in an attack at the Bardo museum in the Tunisian capital Tunis.

The cancellation of holidays to Tunisia could prove costly for TUI and Thomas Cook.

Holidays in North Africa account for about 10 percent of their total passengers, with Tunisia about a third of that.

Shares in the two tour operators have fallen over the last two weeks, hurt mainly by the deadly attack on tourists at Sousse and the cost of cancellations and rebooking.