Tunisia travel alert: Thousands of Britons fly home
The first of thousands of British holidaymakers have returned home from Tunisia after a warning that another terror attack is "highly likely".
The move drew criticism from tourists - and from Tunisia, which said it had done all it could to protect people.
But the UK foreign secretary said it was "too big a risk" not to act.
Philip Hammond said the UK hoped to downgrade its travel guidance "in the not too distant future".
Ireland, where three of the victims were from, has also warned against "all non-essential travel" to the country. Denmark followed suit but Germany and France have not altered their advice.
Between 2,500 and 3,000 British package holidaymakers are believed to be in Tunisia, as well as about 500 independent travellers.
The Association of British Travel Agents (Abta) said its members were aiming to bring customers home in the next 48 hours.
The first British tourists arrived at Manchester Airport shortly after 13:00 BST.
The UK government had stressed there was no new "specific or imminent" threat, but intelligence had led officials to the view that a further attack was "highly likely".
Mr Hammond said the government had been careful not to act in a "knee-jerk manner".
"We now have a much better picture of the emerging scale of the threat as well as a much better understanding of the mitigations the Tunisians have put in place," he said.
Mr Hammond said Tunisian authorities were continuing to hunt individuals suspected of having links to the Sousse attack and an attack in March on the Bardo Museum near Tunis, which left 22 dead, including one Briton.
BBC security correspondent Frank Gardner said counter-terror experts, along with Tunisian authorities, had concluded "that the country faces a very serious threat from an only partially-dismantled terror cell".
He said this means Seifeddine Rezgui had accomplices were still "at large" and they "have probably got access to automatic weapons" and "could quite possibly be looking for new Western targets".
The UK warning comes as the funeral of two victims of the attack - Jim and Anne McQuire, from Cumbernauld - took place in their home town.
The Tunisian prime minister said the country had done "everything we can to protect (British) citizens and their interests, as well as those of all other countries".
Habib Essid said the government would help people leave, but he planned to speak to his British counterpart David Cameron about the change in travel advice.
By Frank Gardner, BBC security correspondent
The government's decision to warn all Britons to leave Tunisia was not taken lightly.
But the large number of British investigators sent out there and working alongside their Tunisian counterparts, concluded that the country faced a high threat from an only partially-dismantled terrorist cell.
In other words, that jihadist accomplices of the murderer Seifeddine Rezgui were still at large, probably with access to automatic weapons and possibly searching for more opportunities to kill western tourists. That threat still exists today.
The recent safety precautions by the Tunisians, what the Foreign Office calls 'the mitigation efforts', while welcomed were not considered enough to meet this threat. Hence the revision to the travel advisory.
But while tourists and expatriates are being brought home, Britain is leaving behind in Tunisia a number of advisors to help the authorities improve their security with the hope that in a few months' time, the decision can be reversed and tourists will be free to return to this much-loved holiday destination.
Nabil Ammar, the Tunisian ambassador to the UK, said the move played into the hands of extremists, adding: "This is what the terrorists want."
One Whitehall official told the BBC it was a "very tough decision" and the government was "working hard with the Tunisians to get to a point in the future where we can reverse this decision".
Foreign tourism accounts for about 15% of Tunisian GDP and European Union foreign ministers are to discuss support for the country at a meeting next week.
Tour operator Thomson and First Choice said it had no customers in Tunisia but was repatriating all British staff.
It had already cancelled all flights to the country to the end of the summer season (31 October) and said customers could change their holiday free of charge to any destination on sale.
It is also offering additional flights to destinations including the Balearic Islands, Spain and Cyprus.
By Rana Jawad, BBC North Africa correspondent
Resignation, rather than relief, was on display in the departure halls of Enfidha-Hammamet airport.
Some British travellers said they knowingly came here after the attack, hoping it would be OK.
Others didn't even know why they were instructed by their tour operators to pack up, ready to be bussed away for a flight - just days after their arrival.
Sun-kissed children appeared none the wiser as their parents fretted over luggage and spoke of their disappointment about the upgraded travel advice - many of them saying they felt safe with the armed security at their hotels.
The arrivals hall was still receiving other European tourists, but in visibly dwindling numbers.
The echoes of a vacant hall - that would usually be welcoming and sending off a throng of holidaymakers at this time of year - are growing louder.
So too are the worries over the fate of hundreds of thousands in this country whose livelihoods depend on tourism.
Thomas Cook, which has cancelled all bookings to Tunisia until 31 October, has two extra flights bringing customers back to the UK.
And a spokesman added customers would be flown back "as soon as we can using third-party carriers and on our 10 scheduled flights over the weekend".
Monarch Airlines said it was arranging to repatriate all customers in resorts back to the UK "as soon as possible".
- Total number of tourists for 2012: 6 million
- Top five nationalities by nights stayed in Tunisian hotels by non-residents (2012 - latest figures available):
- French: 23%
- German: 18%
- Italian: 12%
- Belgian: 6%
- British: 5%
Abta, which has released a Q&A for holidaymakers, said those due to travel to Tunisia should contact the company they had booked through.
Travelling to the country now would probably invalidate travel insurance policies, though most policies would provide cover for those already in the country, it added.
The Tunisian government has increased security in tourist areas, and announced plans for a wall along the border with Libya, to counter the terror threat.
On Friday, officials said Tunisian security forces had killed five suspected Islamist militants.
Tunisia believes the beach attack gunman, Seifeddine Rezgui, trained with the Ansar al-Sharia group in the neighbouring country, though the Islamic State group claimed the attack.
Eight people have been arrested on suspicion of collaborating with Rezgui, who was killed after the shooting spree.
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