Tour operators and agents are continuing to work 24/7 to repatriate customers from Tunisia.
Thomson and First Choice sent 12 flights to Tunisia on Sunday to bring home customers at the end of their holiday and those who wished to return early.
It said its team was working round the clock ‘handling a continuously moving and challenging situation’.
It asked customers to ‘please bear with us’ and said those directly affected by the tragic events in Tunisia were the main priority.
The two hotels targeted by the gunmen – RIU Imperial Marhaba and the RIU Bellevue – are both part of the TUI programme.
The attackers killed 39 people, the majoritiy of them British.
Thomson and First Choice have confirmed that some of the victims were their customers.
"We will continue to do all we can to support our customers as well as their family and friends in resort at this exceptionally difficult time," it said.
It said the whole of Thomson and First Choice are ‘deeply shocked and truly saddened by the events’ and thanked staff on the ground and in the air and the emergency services who are working hard in an ‘incredibly challenging environment’.
Anyone with concerns about friends and families should continue to call 0800 0885372.
Meanwhile, Thomas Cook said it has brought back thousands of holidaymakers who wanted to leave Tunisia earlier than scheduled.
Three repatriation flights operated over the weekend, alongside its regular scheduled flights.
In total, around 3,600 Thomas Cook UK customers have left Tunisia since the attack on Friday.
Thomas Cook opened more shops than usual on Sunday and increased staff numbers in its call centres to cater for increased customer demand.
"We are committed to doing everything we can to support our customers who want to depart, and will continue to closely monitor the situation," it said.
"If there is further demand we will provide additional capacity to ensure all customers who would like to come back to the UK can do so safely and as soon as possible."
Thomas Cook does not feature either of the two affected hotels and none of its customers or staff were involved in the incident.
"However, we do currently have customers staying in other parts of the resort, and our experienced teams on the ground are continuing to offer every support to them and their families at this difficult time," it said.
"Thomas Cook would like to extend its most sincere condolences to the family and friends of those affected."
