Ryanair says biggest-ever strike to ground 600 flights next week

* Cabin crew in Spain, Portugal, Belgium strike

* Action latest in a series of strikes

* Ryanair says willing to discuss demands (Adds Ryanair quotes; details of other strikes; background)

By Graham Fahy

DUBLIN, July 18 (Reuters) - Ryanair is to cancel more than 12 percent of its flights for two days next week in its biggest-ever strike as cabin crew escalate a staff revolt that began among disaffected pilots last year.

The Irish airline, Europe's largest by passenger numbers, said in a statement it expected disruption to the travel plans of almost 50,000 customers traveling to and from Belgium, Portugal and Spain, as it would cancel up to 300 flights per day.

"These strikes are entirely unjustified and will achieve nothing other than to disrupt family holidays," the airline said in a statement.

In the company's largest previous industrial action, Ryanair canceled 30 flights on July 12 when some pilots in Ireland (Other OTC: IRLD - news) held their first strike. Ryanair said it will cancel 24 flights on Friday during the second of three strikes planned by the pilots.

Cabin crew from across Europe earlier this month published a list of 34 demands on July 4, including "a fair living wage", improved sick pay and employment contracts in their own language, based on local rather than Irish law.

Cabin crew have complained that they have to pay for drinking water during flights and must physically report to work when sick to provide written details of their symptoms.

Ryanair, which published a list of cabin crew benefits on Twitter (Frankfurt: A1W6XZ - news) on Wednesday, including pay of up to 40,000 euros per year, said its staff has some of the best conditions in Europe's low-cost airline sector.

"We think are crew are well paid and we think their terms and conditions are good," chief marketing officer Kenny Jacobs told Reuters. "But we will sit and meet with them see what issues they have and consider them. We will keep an open mind."

The airline will consider issues including demands for local contracts by some staff, but the airline needs to maintain its "low-cost, high-efficiency" model, he said.

Ryanair, which flies in 37 countries and carried 130 million passengers last year, averted widespread strikes before Christmas by deciding to recognise trade unions for the first time in its 32-year history.

But it has since struggled to reach agreement on terms with several of them.

German Union Verdi earlier on Wednesday announced it had signed a union recognition agreement for Ryanair cabin crew.

Ryanair decided to cancel flights seven days in advance because it saw little prospect of the unions calling off strikes and wanted to give passengers a chance to rebook, Jacobs said.

The airline, which is due to release financial results for the three months to June 30 on Monday, posted a record annual profit of 1.45 billion euros in the year ended March 31. But it warned profit was set to fall due to higher fuel and staffing costs and flat fares. (Reporting by Graham Fahy; additional reporting by Conor Humphries; Editing by Edmund Blair and Jan Harvey)