A strike by Ryanair's Portuguese cabin crew started on Wednesday, with the low cost airline warning passengers that a small number of flights may be affected and the government ordering workers to deliver a minimum service to avoid chaos.

The five-day strike called by Portugal's SNPVAC union is the first of a wave of planned walkouts by unions in Ireland, Britain and Spain. It comes a year after strikes over pay and conditions forced Europe's largest low cost carrier to cancel hundreds of flights.

Ryanair said on Tuesday a small number of fights from its Portuguese bases might be affected by minor schedule changes and that affected customers had been notified. There are no delays or cancellations reported so far at Lisbon, Porto and Faro airports.

Management say significant progress has been made since last year's strikes, with collective labor agreements concluded with a number of pilot unions throughout Europe.

But SNPVAC said Ryanair had refused to comply with a protocol signed last November, which it said included holiday pay, 22 days of annual leave per year and full compliance with Portuguese parental law.

At the height of the busy summer season, Portugal's government set out a minimum service for Ryanair and its staff to deliver, including a daily round trip between Lisbon and London.

SNPVAC said the minimum service order was "abusive", arguing it undermined the right to strike.

Belgium's CNE and ACV PULS trade unions told members on Tuesday not to comply with a Ryanair request to staff flights affected by the Portuguese strike.

Spain's STICPLA union also sent a letter to its members, advising that they were not obliged to "answer company calls" if they were in their rest time, on days off or on holiday.

Unions representing cabin crew in Spain said their plans for 10 days of strikes next month still stood after more than seven hours of mediated talks with the airline ended on Tuesday without agreement.