Airlines have cancelled scores of flights to mainland Greece and the islands due to a 24-hour strike.
Ryanair has cancelled all of its 90 Greek flights today due to the walk-out, which includes air traffic controllers, and condemned their action.
"We sincerely apologise to all customers disrupted by these unjustified strikes – the fouth such ATC strike in the last three weeks – by ATC unions who are holding European consumers to ransom," it said.
"We call on the Greek Government and EU Commission to take action to prevent any further ATC disruptions. You cannot have a ‘single market’ in Europe but then allow tiny national ATC unions to regularly close the skies over Europe, causing hundreds of thousands of passenger flights to be cancelled or delayed."
It encouraged customers and the trade to sign its online petition ‘Keep Europe’s Skies Open’.
Meanwhile, easyJet confirmed it has cancelled flights to and from Greece and the Greek Islands but did not disclose how many.
"We are doing everything possible to minimise the impact of the strike on our customers, and have taken the decision to proactively cancel a number of flights, allowing customers to reorganise their journeys," it said.
"We will communicate to affected customers directly via SMS using the details provided at the time of booking."
Any affected customers will be offered a free of charge transfer to a new flight or a full refund.
It also advised passengers to allow plenty of extra time to get to the airport and consider alternative transport options where possible, as public transport services will be also affected.
British Airways has cancelled Athens flights and said it will be using larger aircraft and extra Athens services on Friday when airspace is fully reopened.
Olympic Air cancelled 36 scheduled flight sectors yesterday ahead of the walk-out.
The UK Foreign and Commonwealth Office is advising travellers to check with their airline or tour operator before travelling.
"Public services such as hospitals may also be affected," it added.
