Single parent families could be paying significantly more per person to go on holiday than a two adult family.
Research carried out for Sky News has shown that parents travelling on their own with one child could pay up to 28% more for the same holiday as two adults with one child.
Currency firm FairFX compared prices for an all-inclusive holiday at leading UK travel companies, including Airtours, Thomas Cook, First Choice, Virgin and Thomson, and found inconsistencies in pricing, with an increase in cost ranging from 16% to 28%.
Terms and conditions on the travel operators websites also show that many of the free child places and other deals are only valid if two adults are travelling.
This has led to questions surrounding the provision for the two million single parents in the UK, which amounts to around a quarter of all families with children.
Ian Strafford-Taylor, chief executive of FairFX, said the travel industry needs to evolve and make its pricing more flexible.
He said: "I think it's crazy, it's symptomatic of a bigger disease.
"The pricing models they use are from a different century and life and society is changing.
"People go online more and people's family units are changing all the time so holiday companies need to adapt and start offering a discount to single parents."
Single mother-of-two Christine Lewandoski said she found the experience of travelling on her own so miserable and stressful she set up her own business catering specifically for families like hers.
She said: "My first holiday on my own was soul destroying.
"I was surrounded by traditional families and can remember in the evenings in the tent when the kids had gone to sleep and I was there reading a book with my torch and I could see everyone else congregating and having fun.
"I felt like an outsider.
"So I set up single with kids as a hobby at first, and was so surprised by the level of interest.
"We've grown massively and I think it's because people haven't realised the demand, that single parents do need something different, they need the support, the fun and friendship element as well."
In response to the research, the main tour operators, Thomson and Thomas Cook, said they cater for all types of family and they have some offers available for single parents but will work harder to make them more visible.
Sean Tipton, from the Association of British Travel Agents. said the holiday pricing model is based on the majority and comes down to simple economics.
He said: "The reason why single travellers, be it parents or people on their own, pay proportionally more is simply because there is one adult staying in a room and there is a charge for that room and if you are travelling as a couple then in effect you are splitting the bill 50/50."
But there is sign of change. In light of Sky News' investigation, Virgin Holidays has said they're in talks to review their policy on single parents.
A spokesperson told Sky News: "The pricing for our all-inclusive holiday resorts are set by hotel partners and based on two adults sharing a single room.
"However, we do recognise this doesn't suit all customers, such as single parents, and as a result of this enquiry we will look to negotiate better single traveller deals at selected resorts when contracts are due for renegotiation."
