It made the threat as first-quarter profits slumped by 20% amid higher oil prices and rising employment costs, including a 20% pay increase for pilots.
The Irish low-cost carrier said profits fell to €319m (£284.8m) in the three months to 30 June, compared with a profit of €397m (£354.4m) over the same period a year earlier.
Ryainair, which was forced to recognise unions in December for the first time in its 32-year history, is facing strikes over pay and conditions in many of the countries it operates in.
Irish pilots are expected to strike for the third time on Tuesday, while Belgian, Portuguese and Spanish cabin crews are to strike on Wednesday and Thursday.
More than 300 flights have been cancelled from its daily schedule of 2,400 on Thursday and Friday.
In a statement, Ryanair said: “While we continue to actively engage with pilot and cabin crew unions across Europe, we expect further strikes over the peak summer period as we are not prepared to concede to unreasonable demands that will compromise either our low fares or our highly efficient model.
“If these unnecessary strikes continue to damage customer confidence and forward prices/yields in certain country markets then we will have to review our winter schedule, which may lead to fleet reductions at disrupted bases and job losses in markets where competitor employees are interfering in our negotiations with our people and their unions.
“We cannot allow our customers flights to be unnecessarily disrupted by a tiny minority of pilots.”
The airline said that, in the first quarter of its financial year, staff costs increased by 34%.
It has also been forced to cancel more than 2,500 flights due to air traffic control staff shortages in the UK, Germany and Greece – with strikes in France also adding to its costs.
Ryanair added: “Fuel prices have risen substantially from $50 per barrel at this time last year to almost $80 per barrel in Q1. While we are 90% hedged at $58 per barrel, our unhedged balance will see our full-year fuel bill increase by at least €430m (£383.8m).”