The Dublin-based carrier received a refund satisfaction score of just 47% in a survey by consumer group Which?
More than one in five respondents who booked a Ryanair flight that was either cancelled or they were unable to board due to the virus crisis said it took them more than a month to get a refund.
One customer said: “Ryanair is the most awkward airline to deal with that I have ever come across. It seems to be proud of being difficult.”
British Airways was ranked the second lowest of five airlines for refund satisfaction, with a score of 63%.
Some passengers who telephoned the firm reported spending hours on hold only to be hung up on, while others were continuously passed between different departments.
The vast majority of flights were cancelled following the start of the pandemic in spring 2020 as demand for travel collapsed.
Under consumer law, affected passengers were entitled to cash refunds within 14 days, but many airlines were overwhelmed by requests.
Millions of passengers also struggled to obtain refunds for flights which went ahead, but they could not use because of coronavirus lockdowns.
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Jet2.com was ranked the best short-haul carrier for refunds, with 84% of respondents satisfied with how their claim was handled.
One passenger said: “The pandemic has seen Jet2 shine. Its standard of customer care exceeds that of any other low-cost airline.”
In October 2021, Which? surveyed 1,124 members of its online research panel who had experienced a disrupted flight since March 2020.
Of those who had completed the refund request process, 77 had booked to fly with Ryanair and 325 were British Airways customers.
Which? Travel editor Rory Boland said: “Ryanair’s consistently terrible customer service has made it a fixture among the worst performers in our surveys for many years – but the airline plumbed new depths with its handling of Covid refunds.
“BA’s reputation also deservedly took a battering when it took a hard line on refunds for passengers who could not travel because they followed government health guidance.
“Many passengers will not forget how they were treated by companies during the pandemic.
“Covid could still cause disruption to international travel, so we would advise travellers to book with operators that have flexible booking policies and a record of treating their customers fairly.”
British Airways said in a statement: “We’re proud that we were the first UK airline to offer customers the flexibility to amend their plans at the beginning of the pandemic, by providing vouchers that they can use up to September 2023.
“If we cancel a flight we always contact customers to offer a range of options including a full refund. We’ve issued more than 4.2 million refunds and have dealt with more than 3.3 million voucher requests to date.
“However, we know we can do better and we’re working hard behind the scenes, upgrading our phone systems and recruiting more people to deliver a better and faster customer experience that we know our customers deserve.
“We never take our customers’ loyalty for granted and appreciate their patience as our teams work around the clock to support them.”
Ryanair did not respond to a request for a comment on the research.