Furious passengers have swamped Ryanair with complaints and called for a boycott of the budget airline.
Customers scrambling for replacement flights have also accused rival carriers of "cashing in" by hiking fares.
And as aviation authorities claimed Ryanair could be "breaking the law" with its swathe of cancellations — now affecting 740,000 travellers — pilots said the crisis could get even worse.
Under the hashtag #boycottRyanair, customers used social media sites to vent their anger.
Under the hashtag #boycottRyanair, customers used social media sites to vent their anger
Labour MP Graham Stringer led the criticism of boss Michael O'Leary, saying: "He has let passengers down. Nobody can continue with that level of failure." Ryanair pulled the plug on 18,000 flights covering 34 routes on Wednesday.
This followed last week's revelations it was grounding 2,000 flights due to pilot rostering problems. William Shergold, of Sunderland, booked a holiday to Faro from Newcastle.
But when his £99 flight home was cancelled, his holiday was in doubt as a new flight with easyJet costs £306.63.
Aviation authorities claim Ryanair could be "breaking the law" with its swathe of cancellations
He said: "I am unable to pay that. It feels like they are cashing in on our pain with price hikes." But easyJet denied putting up fares, saying: "Our demand-led pricing means the earlier passengers book, the lower the fares."
British Airways said: "This close to a half-term, demand is high for all airlines."
It was revealed yesterday that Ryanair pilots have formed an unofficial union, demanding improved conditions.
Ryanair pilots have formed an unofficial union, demanding improved conditions
One British pilot, with the no-frills carrier for six years, said: "The company thinks it has solved its pilot shortage by cancelling more flights. But the crisis is only going to get worse. Their employment model needs a complete overhaul."
Andrew Haines, of the Civil Aviation Authority , accused the Dublin-based firm of not "making it clear to people their entitlement" to be re-routed and added: "They could be breaking the law."
The CAA has given Ryanair until 5pm today to sort out compensation for all those hit by the cancellations.