The Italian ENAC civil aviation authority cited 178 cases in which Ryanair failed to meet its obligation to assist passengers between April 17 and 22, when airlines cancelled hundreds of flights across Europe as the volcanic ash cloud shut airspace.
ENAC said in a statement that Ryanair passengers stuck in Rome had to be helped by its own staff, members of Italy's Civil Protection agency and employees of the Rome airports operator.
In contrast, almost all the other airlines provided adequate assistance to stranded ticket-holders, ENAC said.
No one was immediately available for comment at Ryanair.
In London, the government said parts of British airpsace might have to close from Sunday until Tuesday because of a fresh cloud of ash from the Icelandic volcano.
Different parts of the airspace including the southeast, where Europe's busiest airport Heathrow is located, are likely to be closed at different times, the Transport Department said.
"Due to continuing volcanic activity in Iceland and prevailing weather conditions, there is -- if the volcano continues to erupt at current levels -- a risk of UK airspace closures," it said in a statement.
Britain's National Air Traffic Service said due to the ash cloud rapidly encroaching on Northern Ireland, a no-fly zone had been imposed for parts of its airspace from 2400 GMT until 0600 GMT on Sunday, although Belfast airport would remain open.
The cloud is likely to be over the west coast of Ireland early on Sunday morning and will cover the rest of the country later in the day, the Irish Aviation Authority (IAA) said in a statement.
The IAA said Sligo, Donegal, and Ireland West (Knock) airports would be open until 7 a.m. BST while other Irish airports would be open until at least 1200 GMT.
"The IAA is organising observation flights for tomorrow to check on the level of ash concentrate," it said, adding that it would provide another update on Sunday morning.
The spread of ash from an erupting volcano under the Eyjafjallajokull glacier in Iceland grounded much of European air traffic for nearly a week in mid-April. Airlines had to cancel around 100,000 flights, stranding millions of passengers.
Since then the ash has periodically forced the short-term closure of parts of airspace in countries across Europe.
British Transport Minister Philip Hammand said on Saturday that from now on five-day -- rather than the previous 18-hour -- ash prediction charts would be made available to airlines and the public on the Met Office forecaster's website.
The response of Ryanair was simply appalling to the Icelandic volcano ash issue. This clearly demonstrates that Passengers cannot trust Ryanair in times of difficulty. Their attitude towards their own customers is so bad that one must really think twice before risking to book with Ryanair, regardless of price.