The budget airline was told it could face legal action by the UK's Civil Aviation Authority (CAA) following a decision to ground 18,000 aircraft until March next year.
These rights and entitlements include full refunds, or re-accommodation onto other Ryanair flights or other comparable transport options with reimbursement of reasonable out of pocket expenses.
"There are clear laws in place, which are meant to assist passengers in the event of a cancellation, helping minimize both the frustration and inconvenience caused by circumstances completely out of their control", CAA CEO Andrew Haines said in the regulator's statement.
More than 700,000 passengers have been affected by two sets of flight cancellations, which the airline has blamed on a mix up over scheduling pilots' annual leave.
Ryanair boss Michael O'Leary last week sought to address the airline's problems with a first wave of 315,000 cancellations.
Ryanair confirmed yesterday that it is to cancel 22 flights a week to and from Dublin, between November 1 and March 24.
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CAA chief executive Andrew Haines told BBC Radio 4's Today programme that "if they [Ryanair] follow through on what they are saying, then they would be breaking the law".
The CAA did not say what action it would take if Ryanair did not comply, but it has the power to take court action against carriers that fail to comply with consumer rights laws.
The airline has until 5pm to sort out compensation for thousands of passengers whos flights have been cancelled.
Among other complaints, the United Kingdom regulator said Ryanair had repeatedly failed to inform passengers about all the expenses they could reclaim in connection with flight changes.
According to the airline, this will affect "less than one daily flight" during the five-month winter period, at the time, they recalled, "many of those flights do not have reservations yet".
As it did when announced the first round of cancellations, the CEO of Europe's leading low-priced airline, once again called for "heartless apologies" to affected customers.