Budget airline easyJet today ramped up the price war in the low cost flights market, amid signs that airlines are struggling to fill their planes.
The group is slashing ticket prices by up to 30% on flights throughout June and July - at the height of the group's busy summer season.
EasyJet's sale comes weeks after rival Ryanair made a similar move, selling one million seats for little or no money, inclusive of taxes.
Both airlines admitted recently they had found it harder to fill seats, although this was at a time when they had also increased capacity.
Ryanair reported its "load factor" - the share of seats filled - fell by 2% in April, while easyJet saw a 3.3% decline.
UK carrier British Airways also posted a fall in its load factor last month, down 2.1%, suggesting the sector's malaise is not just affecting the "no frills" airlines.
Nearly 200,000 fewer passengers went through UK airports last month, according to BAA, and airlines are blaming some of the tough trading conditions on the doubling of air passenger tax at the beginning of the year.
Analyst Andrew Fitchie of Collins Stewart said it was a cause for concern that both easyJet and Ryanair were cutting the costs of tickets during the busy summer season.
EasyJet shares dropped 1%, further extending recent stock market losses when shares fell 7% in one day on the back of warnings of fare pressures.
There are fears that environmental concerns are causing consumers to think twice about flying, although it is also thought that the unusually warm spring weather last month affected demand for overseas flights.
Mr Fitchie said there was hope the issue was a short-term one for the low cost carriers, with both having recently upped the number of planes in their fleets and increased flights, which has also hit their ability to fill seats.
Ryanair increased its capacity by 21%, while easyJet saw an 18% rise on a year earlier.
Mr Fitchie added: "It's not ideal to be making price cuts in summer, but they've both pushed up capacity in a short period of time and now seem to be suffering from indigestion."
EasyJet said the summer sale was "just something we do every year".
The airline is offering nearly a third off some flights to summer destinations leaving between June 1 and July 31, but the offer is only available on bookings made by the end of Friday.