Holiday company Jet2.com has started sending planes to Enfhida to collect people who wish to travel home.
It currently has 1,244 passengers in Tunisia, and flights have already left from Manchester and Leeds Bradford Airports, with the first expected to land at 3pm local time.
There is also a fourth aircraft on standby, and customers are being contacted via text, mobile and email with travel updates.
Priority is being given to people in the resorts of Skanes, Sousse and Port el Kantaoui, and coaches will be leaving this afternoon to take people to Enfhida.
Steve Heapy, CEO of Jet2.com and Jet2holidays said: “Our absolute priority is to ensure the safe passage home of those customers wishing to travel as soon as possible.
“Aircraft are arriving now in Tunisia and will depart this evening.
“Passengers can rest assured that, as required, we will provide onward transport from Leeds Bradford and Manchester Airports.”
UK customers with Tunisia related queries or concerned relatives are asked to call 0113 457 0550, while those in Tunisia should get in contact on 0044 113 387 9501.
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.
Peter Long, joint chief executive of the TUI Group, which includes travel firms Thomson and First Choice, said he was “deeply, deeply shocked” at events in Tunisia, continuing: “Our whole organisation is reeling with pain to see the suffering that is taking place in Tunisia with our customers.”
Speaking outside the company’s office in Crawley, West Sussex, Mr Long said the company had expressed its “deepest sympathies” for the “trauma” that its customers had experienced in Tunisia.
He added: “We are doing our very, very best to look after our guests who are still in Tunisia.”
Nick Longman, managing director of TUI UK, confirmed that some of those who had died were Thomson customers.
The company had 6,400 holiday-makers in Tunisia, he said. Of these, 1,000 had been flown home last night and this morning on repatriation flights, and more would be returning today, Mr Longman said.
The whole of Thomson and First Choice was “deeply shocked and saddened” by events in Tunisia, he said.