Britain has advised all its nationals to leave Tunisia two weeks after an Islamist attack targeting tourists in the coastal city of Sousse left 38 people dead.

Changing its travel advice, the British Foreign Office said the situation has "developed considerably" since the massacre, leading it to believe it is "highly likely" more attacks will follow.

The FCO said:

Since the attack in Sousse, we have been working closely with the Tunisian authorities to investigate the attack and the wider threat from terrorist groups in Tunisia. Although we have had good co-operation from the Tunisian government, including putting in place additional security measures, the intelligence and threat picture has developed considerably, reinforcing our view that a further terrorist attack is highly likely. On balance, we do not believe the mitigation measures in place provide adequate protection for British tourists in Tunisia at the present time and we have therefore changed our travel advice accordingly.

Tunisia Travel advice
The UK Foreign and Commonwealth Office has advised against all but essential travel to Tunisia FCO

The update advised against everything but essential travel to the country, urging those already there who do not have an essential need to remain in Tunisia to leave.

The FCO added:

If you're in Tunisia, you should leave by commercial means. Tour operators are arranging additional flights and will be organising departures for their customers. If you're travelling with a tour operator, you should contact your tour operator's representative in the first instance.

Thomas Cook said all holidays and flights to the North African nation would now be cancelled.

In a statement the travel agents have said:

As a result of the change in the Foreign & Commonwealth Office (FCO) advice to travel to Tunisia, Thomas Cook can confirm that it has cancelled all future bookings to Tunisia up until 31 October 2015.

We have taken this decision to bring all Thomas Cook customers back to the UK as soon as we can using third party carriers and on our 10 scheduled flights over the weekend. Thomas Cook is strongly advising its guests in the country to return on these flights. We are also sending our Specialist Assistance Teams to Tunisia to offer customers additional support in resort.

The Association of British Travel Agents has said roughly 3,000 UK tourists are currently on holiday in Tunisia. Of these 2,000 travelled there with Thomas Cook.

Tunisia, which boasts sandy Mediterranean beaches and is a popular destination with British holidaymakers, had already experienced an outpouring of tourists after jihadi militant Seifeddine Rezgui shot dead 38 people, most of whom were Britons, at two hotels in Sousse.

The FCO said they expected further attacks "including in tourist resorts, and by individuals unknown to the authorities whose actions may be inspired by terrorist groups via social media".

The Tunisian government had taken extra security measures in the wake of the shooting in Sousse..

Earlier in July, President Caid Essebsi declared a state of emergency to give more powers to the army and police, saying the country was "not safe".

In an attempt to salvage the tourism industry that contributes to more than 7% of the GDP, with about 15% of the workforce directly or indirectly linked to sector, security forces were deployed at tourist beaches an outside hotels.

The Sousse attack was the second on tourists in Tunisia in 2015. In March, 21 civilians were killed and one policewoman when gunmen opened fire at Tunis's Bardo Museum.