UK air passengers could be “unnecessarily” paying up to £175 million a year for assigned seats to sit together on flights, according to CAA research.

In an interim report on airline seating charges, published last weekend, the CAA suggests UK consumers are paying between £160 million and £390 million a year for assigned seats – as much as half of it without a need.

The CAA launched an investigation into airline seat pricing in February, with a survey of more than 14,000 consumers which found “having to pay in advance to sit together was one of the biggest concerns for most people”.

It commissioned further research “which found customers were confused” by variations in airline seating polices, and it found the 10 biggest carriers in the UK all received complaints about the practice – with one airline receiving almost 3,000 complaints last year.

The CAA said: “Passengers with reduced mobility may have paid to sit with a carer when the airline would have sat them together for free [and] parents highlighted concerns about being separated from their children, particularly those under 12.”

The regulator concluded: “People might not understand the likelihood of being split up, leading them to pay the optional fee even when the chances of being split up are low.”

It found consumers were likely to pay between £5 and £30 for an assigned seat, but costs could rise to £100.

CAA policy director Tim Johnson said: “Charging for allocated seating has become part of airlines’ pricing strategies, which can impact especially on groups such as those with accessibility needs and those travelling with young children.

“We are also concerned about how easy it is to compare prices and make an informed buying decision.

“We’re proposing a new framework to assess airline seating practices. Alongside the framework, we’ll be working with airlines to explore ways to make prices clearer and more transparent. We’ll also be working with other regulators on the use of allocated seating algorithms.”

Aviation minister Liz Sugg said: “Passengers expect to be charged fairly for services. Allocated seating is clearly a concern.”