Andrew Haines, chief executive of the Civil Aviation Authority (CAA), said he was "furious" with the Dublin-based carrier because it is not telling passengers they are entitled to be re-routed by other airlines.
"They are not making it clear to people their entitlement," Mr Haines told BBC Radio 4's Today programme.
"If they follow through on what they are saying then they would be breaking the law."
A Ryanair spokesman said: "We will be meeting with the CAA and will comply fully with whatever requirements they ask us to."
On Wednesday the airline cancelled an extra 18,000 flights for the winter season in a move that will hit 400,000 customers.
Ryanair is facing a multi-million compensation bill for cancelled flights after it previously revealed it was shelving up to 50 flights a day up to the end of October.
It says the cancellations were brought about because of an error with pilot holiday rosters.
Ten things we love to hate about low-cost flying
Ten things we love to hate about low-cost flying
Is it just us, or is there something seriously nut so about the concept of paying for the privilege of paying? It's a crazy, mixed-up world when you have to pay £10 to use a debit card which costs the airline around 20p to process. Of course, you could apply for one of the cards which are 'free' to use, but they change all the time and take hours to apply for. If we thought about it too hard we'd only ever sit at home and cry.
My dear, the garishness! Bright orange, purple, lurid yellow... it's enough to make anyone long for the days of a discreet livery of navy, red and perhaps a touch of silver. If you weren't feeling queasy before you got onboard, the combination of lime green uniforms and a £10 gin and tonic should do the job. Pass the sick bag – oh no, that's right, there aren't any.
It's all very well paying £3.99 for your flight to Stockholm, but you won't be feeling so clever when you land in a field in the middle of nowhere, surrounded by lakes and forests, with no capital city in sight. Add on the taxes, extra charges and an hour and a half taxi ride into town and suddenly the national carrier flying to the main airport is looking like a pretty good deal. Especially when you factor in the professional service and free prawn sandwich you would have got...
Ah, the joys of the online no-frills airline booking procedure... First off, you have not got a choice: you can only book via the internet (the chances of finding a real live human to book with are about as high you being able to travel on one of the special offer days). You've then got to navigate the site without accidentally hiring a car, paying for golf clubs or adopting a small child. By the time you remember to print out your boarding pass within the correct time period, you're in serious need of a holiday...
The recent story about a certain airline which gave a man a sandwich and a drink after he suffered a cardiac arrest – and then charged him for it – just about sums up the no-frills airline attitude to catering. It's all about the money, money, money. So, three letters for you: B.Y.O.
Seriously, what is up with people who pay for speedy boarding? You haven't spent enough on extra taxes, credit card charges, baggage fees? Sure, whack on another hefty charge while you're at it, just so you can stand in the front of the queue and feel superior. The plane's not going anywhere until the povs at the back of the queue are on too, so save the twenty quid – you'll need it to pay for your cheese sandwich on board.
No, no, we don't really need to take anything with us on our holiday, honestly. We may be going to Norway for two weeks in January, but a toothbrush, t-shirt and a pair of flip-flops will do us just fine. The book, nappies, wet wipes and baby food ? No problem, they'll slip right into the one bag too, that's fine, don't need them at all, yes stick them in the overhead locker miles away from my seat, too. Marvellous.
Remember in the old days when the seat in front of you had a pocket you could stow your bits and pieces, magazines, bottle of water etc, instead of having to strew them all over the floor? They might be saving space and weight, but when your three-year-old starts projectile vomiting and the seat belt sign's on, suddenly a seat pocket full of sick bags makes a lot of sense. Ah, sweet revenge...
Jeez, there's nothing like 29" legroom pitch and a non-reclining seat back to force you to practice your yoga moves. Like human origami, we fold our limbs into unnatural shapes and wonder why we can't feel our feet by the end of the flight. And we're relatively normal! What it's like for a 6ft 7" man, or a 7 month pregnant woman with a 20 month old on her 'lap' doesn't bear thinking about...
OK, when you're paying more for your beer than your air fare, you can't expect silver service, but would a smile hurt? We feel for the cabin crew dealing with leery stag weekenders and bitter businessmen whose companies won't cough up for a proper airline, but it would be nice to be treated slightly more like a human being, less like a walking wallet from whom to extract as much cash as possible in a two hour period. Scratch card, perfume, magazine, £2.50 bottle of water, anyone?