Thousands of tourists rushed to leave Tunisia on Friday after Britain warned another attack was "highly likely", two weeks after a gunman killed 38 foreign holidaymakers at a beachside hotel.

Tunisia's ambassador suggested the warning played into the hands of militants, saying they would feed on the hopelessness that would grip the country if its tourism industry collapsed.

Thirty Britons were killed when Saif Rezgui used a Kalashnikov to gun down tourists at a beach hotel in Sousse on Tunisia's Mediterranean coast, the biggest loss of British lives in such an incident since the July 2005 bombings in London.

"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," British Foreign Secretary Philip Hammond said in a statement.

Hammond said more work was needed to protect tourists and British tourists were instructed to leave. Tour operators put on extra flights to return some of the estimated 3,000 Britons holidaying in Tunisia.

Ireland and Denmark warned their citizens against non-essential travel to Tunisia.

Islamic State militants, controlling large parts of Iraq and Syria, have claimed responsibility for the attack in Tunisia, which is struggling with a rise in Islamist militancy despite its relatively peaceful transition to democracy after the 2011 "Arab Spring" uprising.

Tunisia deployed about 3,000 armed policeman in hotels and on beaches to protect tourists after the worst attack in the country's modern history.

"LACK OF HOPE"

Tunisia is doing all it can to protect British interests, Prime Minister Habib Essid said, adding that he would call British Prime Minister David Cameron to offer assurances about the safety of tourists.

The Tunisian ambassador was more publicly critical.

"This is what the terrorists want," Nabil Ammar, the Tunisian ambassador to Britain, told BBC television on Thursday night. "Hotels have to close and this is an important industry. One of the sources of terrorism is lack of hope."

Tourism accounts for about 7 per cent of the Tunisian economy and is a major source of foreign currency and employment.

Holiday company Thomas Cook said a flight from Tunisia was due to land in Manchester on Friday and smaller rival Monarch said it too was operating a rescue flight. Both plan further flights over the weekend.

Hundreds of British tourists were waiting at Enfidha airport close to the main reports of Sousse and Hammamet.

"It's been difficult and we feel for the Tunisian people because its their livelihood," said Christian, a British tourist leaving for Britain. "But now we have no choice; we have to go home."

Thomas Cook and TUI, which operates Thomson and First Choice holidays, said they had cancelled all future bookings to Tunisia up to the end of October. TUI said it now had no customers in Tunisia.

For TUI and Thomas Cook, Europe's two biggest holiday firms, North Africa account for about 10 per cent of passengers, a third of them in Tunisia.

The two tour operators shares have fallen over the last two weeks, but on Friday TUI was up 1.2 per cent and Thomas Cook was up 1.1 per cent, in line with the market.

Last year about 400,000 Britons holidayed in Tunisia, compared with 425,000 from Germany and 760,000 from France. – Reuters