Holidaymakers are being evacuated back to the UK from Tunisia on special flights after a gunman killed 38 people at a beach resort.
Travel firms Thomson and First Choice confirmed that "a number of fatalities" and those injured are their customers and said around 2,500 people will be brought back to the UK today.
At least five Britons have been confirmed dead in the attack in Sousse.
Ten Thomson flights are scheduled for today, arriving at Manchester, Gatwick, East Midlands and Doncaster airports this morning.
The holiday operators said all its trips to Tunisia for the next week will be cancelled.
In a statement Thomson said: "We would like to extend our deepest sympathies to the friends and families of those involved in this tragic event.
"We are actively working with the families and friends affected to provide support in resort and a special assistance team is en route to Tunisia to assist our customers."
The statement added: "The whole of Thomson and First Choice are deeply shocked and truly saddened by the events and we are grateful to our staff on the ground and the emergency services who are working hard in an incredibly challenging environment."
Thomson advised anyone worried about friends or family to call 0800 0885372.
Customers can change bookings to Tunisia until July 24, the company said.
Thomas Cook said it has arranged for an additional aircraft to fly to Tunisia today, which would be used to support its operations to accommodate customers wanting to return home as soon as possible.
The company is offering customers due to travel to Tunisia up to and including July 4 the opportunity to cancel or amend their holiday free of charge.
For those travelling from July 5, it was offering to amend bookings to Tunisia free of charge for holidays departing up to and including July 31.
The first wave of six morning flights from Enfidha to Manchester arrived early today.
A team of Thomson representatives were on hand to greet passengers as they entered the arrivals hall at the airport's Terminal 2 building.
Some were clearly distressed as they were led away by staff within the airport, with one middle-aged woman in tears.
Two ambulances were parked outside the arrivals hall.
The airport's co-ordinating chaplain, the Rev George Lane, was on site, along with a visible police presence.
Police officers asked reporters not to approach passengers and told them - under instructions from press officials - to leave the grounds of the airport.
It was a bloody day: a wave of attacks across three continents within a matter of hours, leaving more than 60 dead and stoking fresh fears about the threat posed by jihadists claiming affiliation with or inspired by Islamic State, the militant group also known as Isil.
The fact that there are still tourists to attack in Tunisia tells its own story. Ever since it became the birthplace of the Arab Spring in 2011, the tiny north African nation has been the only country in the region to enjoy anything approaching stability after the overthrow of its resident dictator.