Families face paying up to double the price for a package holiday to popular destinations as soon as the school holidays begin, a survey has found.
A survey of 79 one-week package holidays for a family of four at a four star hotel in Tenerife, Majorca, the Costa del Sol and the Algarve found that prices increase by up to 115% compared with the same trip taken two weeks before schools close for the summer.
Travel money provider FairFX, which carried out the research, said it showed how travel companies "exploit" parents by hiking prices as soon as schools in England and Wales break up.
On average, the cost of holidays rose by 35% but the investigation also found that the biggest hike was a 115% increase in the cost of a Thomas Cook package holiday to Majorca from London and a 104% increase in a First Choice holiday to the Costa del Sol flying from Birmingham.
The study found that Leeds was the most expensive regional airport on average for all the destinations with price hikes of around 34%.
Package holidays from Birmingham saw the highest regional average price increases at 43%.
Those flying from London saw average prices become 45% higher after the start of the school holidays.
The study follows a similar investigation in February, also by FairFX, that found air fares for half-term flights were up to eight times more expensive than at other times.
The figures come as many parents make plans for summer holidays abroad, but face fines if they take their children out of school for unauthorised absences.
FairFX chief executive Ian Strafford-Taylor said: "It seems parents are being held to ransom by travel operators when it comes to summer holidays.
"A week in the sun allows families to spend precious time together, but when parents face paying double the original price this experience comes under threat."
A Thomas Cook spokesman said: "Customer demand is always really high during the school holidays, which can unfortunately lead to an increase in costs for families. This is particularly true for a destination like Majorca which is more popular than ever this year.
"However, we do everything we can to give our customers the best value, and we'd always encourage them to book as early as possible so that they can secure the best deal for them. There are still some fantastic, great value deals out there to be had."
A First Choice spokeswoman said: "Hotels increase their prices during high season, therefore this has to be reflected in the pricing of our holidays. Thomson and First Choice offer exceptional value for money and, wherever possible, we offer discounts to customers throughout the year, even in peak periods, as an incentive to book.
"We encourage our customers, particularly those that need to travel during the school holidays, to book early. This allows them to not only secure the property and destination of their choice while there is still availability, but also to take advantage of our many early booking discounts, which include free child places."
An ABTA spokeswoman said: "Price increases in holidays are driven by the economic realities of supply and demand. Where there is pressure on flights and accommodation, or higher demand, prices will typically increase, and where there is less demand, prices will decrease.
"The UK has one of the shortest school holiday 'windows' in Europe - just six weeks - which means there is very intense demand during this period.
"ABTA has called for the education system (Government, local authorities and schools) to stagger school holiday dates by area or region, a system that already works well on the continent.
"This would help to flatten out the extreme spikes caused by intense increases in demand, particularly in July and August, and encourage more travel in June and September when demand is lower and the weather in Western Europe is also ideal for families with children."