A leading legal expert has declared that air passengers are entitled to refuse to pay the increase in duty on airline tickets, which comes into force today.

Travellers who have already bought their tickets face demands of up to £40 in air passenger duty when they check in today, after Gordon Brown announced in December that the duty would be doubled.

One industry source said that the prospect of passengers refusing to pay the extra money was a nightmare. Critics, including airlines, accused the Chancellor of introducing a retrospective tax.

Airlines considered taking legal action, but most will pass on the extra costs, which total £1 billion a year, to passengers, even though it is the airlines who are legally required to pay the tax. Virgin, Ryanair and Easyjet are demanding that passengers pay the increase and British Airways has said that it will absorb the cost.

Adrian Jack, a leading commercial barrister commissioned by the Conservatives to give a legal opinion on the new tax, said that it had no legal basis because the Chancellor had not passed it through the House of Commons. He said that the tax would not be legal until it was approved in the Finance Bill, expected to be passed in April.

Airlines insist that the small print allows them to pass increases in tax on to passengers, but Mr Jack said that small print does not legally apply if the tickets were bought before December 7, the date of Mr Brown’s announcement.

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