The row between the board of easyJet and its biggest shareholder escalated yesterday as Sir Stelios Haji-Ioannou questioned the low-cost airline's ability to finance aircraft purchases and said that it would be loss-making this winter.

The entrepreneur's latest salvo at the board comes after Sir Stelios's decision not to sign-off the airline's figures amid a dispute over the way in which the acquisition of GB Airways was accounted for.

In a letter to the board, Sir Stelios said that certain aspects of the accounting treatment did not reflect “current commercial realities and the macro-economic climate”. He also questioned the value put on landing slots at Gatwick acquired with GB Airways a year ago and called for the value of aircraft from the GB deal to be written down in the accounts.

The airline's founder also used the letter - his first formal public statement on the boardroom row - to call for the company to agree to pay a dividend by 2011 if markets and company finances allow; and to make clear that he was not trying to snatch the company chairmanship from Sir Colin Chandler.

EasyJet, in which Sir Stelios raised his stake to 27 per cent last week, said that it would defer four deliveries of new aircraft in 2010 and would continue to monitor the pipeline of deliveries quarterly.

The low-cost airline is contracted to take delivery of 109 jets from Airbus in the next four years at an undisclosed cost that is likely to be somewhat lower than the $5.1 billion (£3.3 billion) list price.

Plans to take delivery of 36 aircraft in 2009 appear to be unchanged, despite predictions of flat growth in 2009. A further 30 are due to be delivered in 2010.

EasyJet said that it was financially strong, with £863 million of cash and undrawn facilities of $1.1 billion. However, the airline will spend $1.2 billion on aircraft in the next 18 months.

Sir Stelios is understood to have serious concerns about the airline's ability to secure this financing. “I would like to place on record that I believe that with careful cash management and, in particular, more prudent capital expenditure, easyJet and its shareholders will be the winners in European short-haul aviation. We must focus on cashflow forecasts and not on carrying more passengers,” he said.

Despite falling oil prices, easyJet has said that margins are likely to be diluted over the winter and it has removed 24 routes in anticipation of falling passenger numbers. The airline lost £57 million last winter and there are concerns that this year's losses will be even higher.

Andy Harrison, the chief executive, tried to play down the row, saying that in his 18 years on boards of public companies, there had never been any shortage of strong views on company strategy. “No one should be surprised that there is a big debate on how we proceed in this economic climate,” he said.

Over the next few days, Mr Harrison said that he and Sir Colin would talk to easyJet's other large shareholders, which include Schroders, Standard Life and Fidelity, about the details of the dispute.

He denied that he would look to appoint non-executive directors loyal to him if Sir Stelios succeeded in having two easyGroup directors appointed to the board.

EasyJet's full-year figures to September 30 showed the effect of high oil prices, with underlying pre-tax profits down from £191 million to £123 million. The shares fell 25p, or 9per cent, to 251p.

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