The victims, all from the Newcastle upon Tyne area, had flown out to the popular coastal resort of Sousse and arranged their own accommodation.
Two Tunisian men are also said to have died in the house, with the deaths attributed by the Foreign Office to a poorly ventilated water heater that allowed a build-up of the lethal odourless gas in the house.
Last night the apartment's owner, a Tunisian man, was said to have been arrested in connection with the deaths and, although not suspected of foul play, he is expected to be held liable by local officials for breach of building regulations.
Friends named those who died as Sylvia Smith, her daughters Alana, 13, and Stacey, 12, and Michelle Thompson, a family friend.
The party had flown out on an open-ended ticket on January 16 and it is not known when they were due to return. Their bodies were discovered on Wednesday after the alarm was raised by neighbours.
Michelle had worked at the Dunston Excelsior workingmen's club on Tyneside for the past few months. The steward's wife, Cathy Nelson, received a telephone call from Michelle's sister, informing her of the deaths.
Mrs Nelson said: "Everyone is still in shock as Michelle was extremely well liked by customers and staff alike. She was a lovely girl and will be missed by everyone at the club.
"Michelle and her friend Sylvia and the sisters Alana and Stacey had only flown out on holiday shortly before."
The two girls were pupils at Tyneside's King's Meadow School but all calls are being referred to Gateshead council's education department, which is referring requests for information to the Foreign Office.
It said that post-mortem examinations had been completed on the victims and that carbon monoxide poisoning had been confirmed. John Jeffries, British consul in Tunis, said: "Police do not suspect foul play but have said they will be prosecuting the owner of the house although they have not said on what charges."
Robin Twyman, from the foreign Office, said it was talking with relatives on Tyneside. "The British consul, who arrived yesterday, is awaiting instructions to fly back the bodies," he said. "The group were found by police on Wednesday evening after they were alerted by a neighbour."
A spokeswoman for Thomson Holidays said: "It is with deep regret that we can confirm these four deaths. The four people involved travelled out to Tunisia on January 16 on a Britannia flight and then booked their accommodation independently. Our deepest sympathy goes out to their family and friends."
Sousse, a lively mix of North African culture and sophisticated nightlife, is currently enjoying a tourist boom.
Tunisia's national tourist board said visitors had increased by 11 per cent on last year as Europeans flock to enjoy its colourful sooks and pleasant beaches. But it has also heralded a massive construction drive as leisure companies seek to cash in by building new hotels, apartments and villas.Reuse content