LONDON (AFP) - Britain on Thursday advised its nationals in Tunisia to leave, warning that local authorities could not provide "adequate protection" after last month's beach massacre that left 30 Britons dead.

"While we do not have any information suggesting a specific or imminent threat, 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," Foreign Secretary Philip Hammond said.

"We judge that more work is needed to effectively protect tourists from the terrorist threat," he told the BBC, adding that it was "right and prudent" to change the travel advice.

The Foreign Office now advises against "all but essential travel" to the north African country, adding: "If you're in Tunisia and you don't have an essential need to remain you should leave by commercial means."

"On balance, we do not believe the mitigation measures in place provide adequate protection for British tourists in Tunisia at the present time," it said.

At the time of the June 26 attack, there were about 20,000 British holidaymakers on package holidays in the country, according to ABTA, Britain's largest travel association.

Many people have cancelled their trips to Tunisia, however, and a spokeswoman for ABTA said Thursday that an estimated 3,000 British tourists remained.

Within minutes of the travel advice change, tour operators Thomson and First Choice said they had cancelled all flights to Tunisia for the rest of the season, until Oct 31.

"Thomson and First Choice have no customers currently in Tunisia, but as a precautionary measure we have taken the decision to repatriate all British Thomson and First Choice staff currently working in Tunisia within the next 24 hours," they said in a statement.

'NUMEROUS' TERRORIST PLOTS

Tunisia stepped up security around tourist sites in the wake of the attack, the second on tourists in three months, both of them claimed by the Islamic State in Iraq and Syria militant group.

The gunman, 23-year-old student Seifeddine Rezgui, was killed by police at the scene, but authorities made a number of arrests of people they say were connected to the attack.

On Saturday, the government declared a state of emergency for 30 days. Prime Minister Habib Essid told parliament on Wednesday that Tunisia "was facing numerous terrorist plans to destabilise the country".

Hammond said he had spoken to the Prime Minister earlier on Thursday "and underlined our commitment to working with Tunisia to help them tackle this threat".

But the travel advice makes clear that Britain does not have faith in Tunisia's ability to keep tourists safe.

"The Tunisian authorities have increased their security measures but have also acknowledged the limitations in their ability to counter the current terrorist threat," the Foreign Office said.

Last month's rampage at the Mediterranean resort of Port El Kantaoui, near Sousse, killed 30 Britons, three Irish nationals, two Germans, one Belgian, one Portuguese and a Russian.

The attack was Britain's worst loss of life in a terror incident since the 2005 London bombings, and was marked last week by a minute's silence across the country.

The Foreign Office had previously advised against travel to certain hotspots in Tunisia, including near the borders with Algeria and Libya, but said tourists could go elsewhere.

Travelling to a country against the advice of the Foreign Office invalidates a tourists' insurance, ABTA said.