THOUSANDS of air passengers will face paying extra to board their flights today when the controversial "green" increase in Air Passenger Duty (APD) comes into force.

Although airlines have already collected the tax rise from many passengers, others will turn up at airports to find they have to stump up the additional cash.

Chancellor Gordon Brown announced a doubling of APD during his pre-budget statement in December and said it would come into force on February 1.

Tickets bought since the announcement have included the tax rise in the price but passengers who had already booked flights have faced uncertainty over paying extra.

British Airways decided to absorb the tax hike at an estimated cost of £11m, meaning that passengers who booked before the announcement have not been asked for more.

But most other airlines have chosen instead to pass on the increase, which they may not then have to pass on to the Government.

Travellers booked on some low cost carriers have seen the extra charge added to their credit card bills. Others were told of the increase by email.