Seventy British holidaymakers have fallen ill at holiday resorts in Menorca and Majorca in Spain.
At least 50 people are believed to have fallen ill at the Club Mac Alcudia inclusive hotel in Majorca, while on the neighbouring island of Menorca, at least 20 more people have fallen ill at the three-star Stil Victoria Playa hotel.
Emma Watkins, 32, said their holiday at Mallorca's Club Mac Alcudia, booked through Thomas Cook, had been a disaster and her daughter had ended up in hospital.
Speaking to the Daily Mail, she said: "It was our first family holiday ever, the first time we had gone away with the kids and it was so traumatic.
'On the first day my three-year-old daughter was sick and then my son and then the rest of us. People were being sick everywhere, there were children throwing up all over the place, on the stairs, everywhere.
'We had to take my daughter Faye to hospital and she was really scared, she had to be injected and she had a drip, it was awful.
"The whole thing was a disaster, we stayed in our room most of the time and we lost our holiday and our money."
Other holidaymakers have complained of undercooked food and sub-standard hygiene at both hotels.
Anne Thomson, head of travel law at Your Holiday Claims, told Chronicle Live: "Many people have been confined to their rooms or required hospital treatment and had their holiday ruined due to suffering symptoms of profuse sickness, diarrhoea, fever and stomach cramps.
"Despite numerous complaints that have been made to the holiday representative's onsite, it would appear that a full investigation into the cause of the outbreak has yet to be undertaken."
But Thomas Cook and Thomson say this is not the case.
A Thomson spokesman told the Independent: ""We are aware of a very small number of cases of sickness in Victoria Playa hotel, Menorca, with symptoms experienced similar to a virus.
"Our Holiday Advisors in Menorca have been supporting and advising any customers who have reported that they are unwell.
"No new cases have been reported within the last week.
"A thorough investigation has been conducted by an independent health and safety consultant, who has also confirmed that the correct containment procedures are in place.
"Thomson closely audits all the resorts to which we operate to ensure that health and hygiene levels are maintained."
And a Thomas Cook spokesman told the Daily Mail: "We take health and hygiene issues very seriously and maintaining the safety of our guests is our number one priority. In early June of this year, a very small number of our customers staying at the Club Mac Alcudia in Majorca reported a stomach upset, usually lasting approximately 24 to 48 hours.
"We are aware that a statement was recently issued by a no-win, no-fee lawyer, which contains as yet unsubstantiated allegations with regards to the nature of this illness.
"Preliminary indications from the independent hygiene consultant and our own investigations suggest that this was of a viral nature."
British couple receive £13k payout after horror holiday hotel sickness bug in Turkey
Victims of hotel sickness bug get share of £500k payout
63 diners hit with vomiting bug at 'world's best restaurant'