The gender pay gap widened at almost half of British firms in 2018, according to an analysis of new data.
U.K. firms with at least 250 employees were legally required to submit their pay gap data to the government by Thursday.
The data, available from the U.K. government website, showed that almost 80 percent of firms had a gender pay gap that favored men, while 14% of firms paid their female employees more than men.
On average, men were paid 11.8% more than women in the U.K. last year, based on a list of companies with more than 250 employees.
The median pay gap is calculated by comparing the discrepancy in pay between a company's middle-ranking male and female workers.
According to a BBC analysis of the data, the pay gap widened at 45% of British firms in 2018, while 7% of companies reported no change.
Ryanair was ranked among the worst performers, with a median pay gap of 64%.
The company said in its latest gender pay gap report that its numbers reflected a low number of female pilots.
"Although it is a global feature of the aviation industry that more males than females choose to enter the pilot profession, we continue to see a welcome increase in the number of female pilot applicants," the airline said. "In the past year we have doubled the number of female pilots in the U.K. and are committed to continuing this growth in the coming years."
Other companies with gender pay gaps above the 11.8% average included Credit Suisse, which had a gap of more than 29% in the U.K., and luxury fashion brand Chanel, which had a median pay gap of more than 13%, according to the data.
In Credit Suisse's latest gender pay gap report, published in March, U.K. CEO David Mathers admitted the bank's pay gap was "far from good enough."
"The principal reason for this disparity has been well covered in the media over the past 12 months: there are not enough senior women in financial services organisations," he said. "As an industry, we have to work together as a whole to effect change and recognize that it is only by every employer playing their part that permanent change will begin to shine through."
A spokesperson for Chanel told CNBC via email: "Chanel continues to be focused and committed to closing the gender pay gap and promoting an equal and inclusive environment for women and men through a number of initiatives."
"In contrast with many other companies, women hold 40% of Chanel's executive positions," the spokesperson added. "Chanel guarantees that men and women in the company who undertake the same or similar roles get equal pay."
Clothing retailer Primark, which employs more than 20,000 people in the U.K., reported an equal hourly wage for its middle-paid male and female workers. In its 2017 gender pay gap report, the company acknowledged that while it employed more female retail assistants, the majority of its senior positions were held by men.
Spokespersons for Ryanair and Primark were not immediately available for comment when contacted by CNBC.