Thousands of Brits will have their travel disrupted, as the mass walkout comes at peak holiday season, with schools having just broken up for the summer.
Meanwhile, Italian cabin crews will strike today, but the budget airline says flights will not be affected.
Ryanair said those travelling have been contacted and “90 percent of affected passengers have been transferred” to another flight.
The Irish company said: “Ryanair has added extra Customer Service staff to help re-accommodate/refund these customers over the next seven days.
“Any Ryanair customers travelling to/from Belgium, Portugal or Spain on Wednesday or Thursday, who have not received an e-mail or text SMS, should expect that their flight will operate as scheduled and should check-in to their departure airport a normal.”
The strikes follow disputes over working conditions and pay, as living costs increase, but a Ryanair spokesman has called the walkout “unnecessary”.
Chief marketing officer Kenny Jacobs said: “Ryanair sincerely apologises to our customers for these disruptions which we have done our utmost to avoid.
“Given that Ryanair cabin crew enjoy great pay-up to €40,000 per year, industry-leading rosters (14 days off each month), great sales commissions, uniform allowances and sick pay, these strikes are entirely unjustified and will achieve nothing other than to disrupt family holidays, and benefit competitor airlines in Belgium, Portugal and Spain.”
But cabin crew unions have hit back, saying there is a disconnect between what Ryanair says employees get and the reality.
Sitcpla cabin crew union spokesman Antonio Escobar said: “In Italy, some 100 Ryanair cabin crew sent Ryanair a letter to claim the difference between the €17,000 they’ve earned and these €40,000.
“The large majority of cabin crew don’t have a base salary, which means if you don’t fly you don’t get paid.”
Unions want the airline to give contractors the same work conditions as its own employees.
Mr Escobar said: “On board the same plane, for instance, a Madrid to Charleroi flight, you will find four cabin crew members doing the same job, but none will be earning the same.”
This is because some are employed by Ryanair and others by temporary working agencies.
Unions are also calling for staff to be employed according to the national legislation of the country they operate in, rather than that of Ireland, which they currently are.
Earlier this month, a Dutch appeals court ruled that Ryanair employees based in the Netherlands must be covered by Dutch law.
Portuguese SNPVAC cabin crew union is anticipating “strong participation” in the strike, according to spokesman Bruno Fialho.
He said the joint strikes would “stop Ryanair from using other crews to replace those striking, as was the case during the last strike in Portugal”.
These cabin crew walkouts follow three Irish pilot strikes this month, the last of which took place yesterday.
More strikes will follow, with 60,000 ground staff in Spain announce their plans for industrial action in August.
Ryanair said it expects yet more strikes over the summer “as we are not prepared to concede to unreasonable demands that will compromise either our low fares or our highly efficient model”.
The company on Monday reported first-quarter profit down more than a fifth due to higher fuel costs and pilot salaries.
