Train companies have been "deliberately misleading" passengers over the availability of cheaper advance tickets during busy holiday periods to lure in customers, a transport union claimed.

A survey by the Transport Salaried Staff Association (TSSA) said not one ticket was available at the cheapest advertised fare on the busy East Coast Main Line route between Glasgow and London at the start of Glasgow's traditional holidays in July.

The union said although the National Express East Coast company was advertising advance bargain tickets of £15.70 for travel on July 17, 18 and 21, when it tested this when advance sales opened to the public two weeks ago, none was available.

The claims have been vehemently denied by the company, which said yesterday it made tickets available to the public. However, Gerry Doherty, TSSA general secretary, said: "This is proof positive the rail companies are deliberately misleading the travelling public when they claim cheaper tickets are available if you only book far enough ahead.

"This is clearly not the case on one of the country's busiest routes - National Express's east coast line. Not one cheapest bargain' ticket was available, either through the booking office or internet.

"These so-called deals are a mirage, aimed at luring customers in and then forcing them to pay higher fares. You expect that sort of con trick from street traders, not reputable public companies."

Yesterday, tickets were available on National Express East Coast service between Glasgow and London for cheaper than the bargain ticket the union said was not available two weeks ago. A single from Glasgow to London was available on July 18 for £14, with a return ticket coming back on July 21 costing £32.

On Virgin Trains's website yesterday, a Glasgow to London ticket was offered at £15 and Edinburgh to Birmingham for £18. When trying to book tickets for July 18, however, a Glasgow to London single cost £18.50, while the cheapest from Edinburgh to London was £30.

Robert Samson, passenger link manager with consumer group Passenger Focus, said: "Rail companies use cheap tickets to get people on the quieter trains. It can be a confusing system for passengers and we need more transparency on how passengers can take advantage of these cheap ticket offers."

National Express East Coast spokesman Alan Hyde said: "This allegation is untrue and it's disappointing that TSSA didn't bother to check their facts. The reality is that our cheapest tickets were available and sold on our Glasgow- London route over the three stated days in July.

"Most people recognise the need to book ahead. It's hardly surprising the lowest-priced tickets sell very quickly."