"If you're in Tunisia and you don't have an essential need to remain you should leave by commercial means," the updated UK travel advisory issued Thursday said.
According to ABTA
, which represents British travel agents and tour operators, there are about 3,000 British tourists in Tunisia with its members, with a smaller number thought to have traveled there independently.
"There's probably around 3,000 British holidaymakers out in Tunisia at the moment compared to 20,000 or 25,000 that we would normally expect at this time of year," ABTA spokeswoman Gillian Edwards told CNN.
The latest warning will be another blow to Tunisia's beleaguered tourism industry
-- already struggling to rebound after the upheavals of the 2011 Arab Spring
. Many Tunisians' jobs rely on tourism, which accounts for some 15% of annual gross domestic product, according to a report
from the World Travel and Tourism Council.
"This attack is obviously going to have a devastating impact on the industry, and on the many jobs of Tunisians as well, so it's a tragedy not only for the British victims of the attack but also for the Tunisians," Edwards said.
Ireland, which lost three citizens in the Sousse attack, also upgraded its travel advice Friday to warn against all nonessential travel to Tunisia in light of increased security concerns.
Relatively few Irish visitors are thought to be in the country at present, Foreign Affairs Minister Charlie Flanagan said, since direct flights from Dublin to Monastir, Tunisia, were canceled this month.
UK: Tunisian security measures insufficient
Extra flights are being provided to bring home UK visitors from Tunisia's resorts, including by Monarch Airlines.
Tour operator Thomson and First Choice said it no longer has any vacationers in Tunisia but has canceled all outbound flights to Tunisia up to October 31.
They are also pulling out all British staff from Tunisia within 24 hours, a statement Friday
said. All 30 Britons killed last month were customers of Thomson and First Choice.
The UK Foreign Ministry acknowledged additional security measures the Tunisian government put in place after the attack, but it said they were insufficient to protect British tourists.
"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," UK Foreign Secretary Philip Hammond said.
U.N. warning about Tunisian fighters
A U.N. working group on the use of mercenaries warned Friday that urgent measures were needed to stop the flow of Tunisians flooding to join Islamist extremist groups in Iraq and Syria.
"The number of Tunisian foreign fighters is one of the highest among those traveling to join conflicts abroad such as in Syria and Iraq," said human rights expert Elzbieta Karska, who heads the expert group, after an information-gathering trip to Tunisia.
According to information received by the group, some 4,000 Tunisians are in Syria, between 1,000 and 1,500 in Libya, 200 in Iraq, 60 in Mali and 50 in Yemen. Some 625 who have returned from Iraq
are being prosecuted, the group said.
The majority are young, ages 18 to 35, but while some are from poor backgrounds -- for whom the promise of financial gain may be a factor -- there are also middle-class and wealthy recruits. The group found evidence that women as well as men are being lured to join extremist groups abroad, some for ideological reasons and others to join family members.
"Sophisticated travel networks operate to take recruits across the porous borders, and sometimes through areas where trafficking in people and illicit goods may not be effectively controlled," Karska said.
"Testimony has documented that the routes taken entail travel through Libya, then Turkey and its border at Antakya, and then Syria
President: 'Terrorism is spreading'
Over the weekend, Tunisian President Beji Caid Essebsi declared a 30-day state of emergency, giving the military and police more authority to combat terrorism and placing restrictions on such rights as public assembly.
That decision, too, was in response to the beach attack.
"Terrorism is spreading," Essebsi said. "I believe, and I say this plainly and clearly: If this were to happen, which happened in Sousse, if this were to happen again, the country would collapse."
has claimed responsibility for the attack, but it is unclear whether the Islamist group had any direct role in it.