The cost of sitting a baby on your lap could cost more than buying their own seat due to soaring charges by airlines.

Research by travelsupermarket.com shows that infant ticket prices on low-cost carriers have increased by a third since last year, with the cost of taking a child under two on a European flight with either Jet2 or easyJet rising from £15 each way in 2010 to £20 each way in 2011.

The report shows it is possible to pay as little as £9.99 for someone to travel with you on a budget flight from the UK to mainland Europe but no less than £20 for an infant on your lap.

A child under the age of two is allowed to sit on an adult’s lap during the flight, but airlines still charge for this even though most do not have a hold or hand luggage allowance.

Travelsupermarket said parents who instead book an adult seat for their infant would benefit from a second hand luggage allowance, as well as the additional seat.

Some airlines’ terms and conditions indicate this might not be allowed.Jet2 and Ryanair will not allow parents to book an adult seat for an infant in order to get the extra baggage allowance. However easyJet says it will allow parents to book their infant into an adult seat and pay the adult rate if that price is less than the fixed-rate infant fare.

That comes with the added luxury of more space and the same hand luggage entitlement as any other adult.

However, babies under the age of two are required by law to sit on an adult’s lap and be strapped in place during take off and landing.

Bob Atkinson, travel expert from travelsupermarket.com, said: “The price with a budget airline may seem attractive but it could end up being more expensive once you pay for an infant seat and for excess baggage.

“The best advice is to check the small print. If there is more than one carrier going to your destination, check which has the best policy and price for travelling with infants.

“If an airline is offering a discounted deal, it could be cheaper to book a seat for your baby and ensure they are on your lap during take-off and landing to satisfy legal requirements.

“Not only will this save you money but the extra space will also make the flight a bit more comfortable, plus you can take advantage of an additional hand luggage allowance.”