The European Commission has increased pressure on airline companies selling tickets on the web to make them comply with rules safeguarding consumer rights.

The Commission began investigating travel websites in September 2007. At the time it found one in three European-based airline ticket websites were ripping off consumers.

The biggest problem found by the investigation was that six in 10 of the sites looked at gave misleading pricing details. Other breaches of consumer law included misleading advertising and unfair practices such as pre-checked boxes for additional paid-for services such as insurance.

The Commission set new rules and said in December that although six in 10 websites have corrected irregularities, too many airlines are still failing to comply with the listed obligations.

Consumer Commissioner Meglena Kuneva said in September 2007 that it was unacceptable that one in three consumers was being ripped off or misled and confused. With the latest report she sent a warning to the airlines.

"My message to the airline industry is very clear. Full compliance with EU law is an obligation, not an option,” she said.

Commissioners have now written to airline representatives setting out 26 consumer rights that all websites selling air tickets must respect.

The letter requests that members of the airline associations amend all websites to ensure they fully comply with several consumer laws. It also reiterates that optional price supplements must be accepted on an 'opt-in' basis.

The letter cites various laws that apply to the sale of air tickets, including the Unfair Commercial Practices Directive, the Unfair Terms in Consumer Contracts Directive and a regulation relating to air carrier liability. It also cites a new regulation on common rules for the operation of air services in the EU, which came into force on 1 November.

The Commission said it will publish a compliance report in May 2009.