Trading standards officers are cracking down on what they describe as 'virtual reality pricing' by some tour operators. There are concerns that fluid pricing - whereby prices go up or down depending on availability and demand - is becoming widespread among the major holiday companies. An investigation is under way to find out whether certain tour operators are breaking the law by printing misleading brochure prices.

Thomson brought the issue into the spotlight by printing a warning in its 2001 brochures that prices may go up or down, prompting concern that other operators will follow suit and make brochure prices redundant.

Trading Standards Officer Bruce Treloar said: 'We want to avoid a situation where people are going into a travel agent with a brochure to book a holiday and being charged £50 more because the operator has decided to shove the prices up.'

The Package Travel Regulations state that if a brochure is produced it must give an accurate price. Legally, operators can add a surcharge after the holiday has been sold for unforeseen extras such as fuel charges, fluctuating exchange rates and airport taxes. All other extras, including flight sup plements, should be included in the brochure price.

Treloar said operators were not always upfront about these last-minute price hikes, occasionally disguising them as 'flight supplements' or selling packages at a higher price under another name or brand.

Tour operators say they are just responding to market forces: the more demand there is for a holiday, the more people will have to pay. A Thomson spokeswoman said: 'We are living in the age of the internet and customers have come to expect real-time pricing. The whole e-commerce culture is built on "what's it worth now".

'Brochures struggle to keep up with this and by printing the warning we are just being honest and open about a practice that is widespread.'

Operators are looking for ways around the problem, one of which is to stop printing brochure prices altogether.

Airtours managing director Richard Carrick said next year's brochures could feature price bands which would give a minimum and maximum price.

'Brochures are out of date from the moment they are printed. We owe it to customers to find a better way of indicating the price of a holiday,' said Carrick.