Martin Sorrell, head of advertising and public relations giant WPP, is reportedly the highest paid chief executive among FTSE 100 companies, taking home £48 million ($65 million)
Bosses at Britain's largest public companies will have on average made more money in the first four days of 2018 than typical workers earn annually, according to independent calculations released Thursday.
The High Pay Centre, a think tank, and human resources body The Chartered Institute of Personnel and Development (CIPD), have dubbed January 4 "Fat Cat Thursday" to mark their findings.
The organisations report the mean pay of chief executives at companies listed on the London Stock Exchange's benchmark FTSE 100 index will pass employees' median yearly salary of £28,758 (EUR32,328, $38,861) during the day.
Although they note executive pay has fallen amid "modest" restraint by company boards, the study authors warn the pay gap between top and average workers "remains wide".
"While it was encouraging to see a tiny amount of restraint on pay at the top of some FTSE 100 companies last year, there are still grossly excessive and unjustifiable gaps between the top and the rest of the workforce," said Stefan Stern, director of the High Pay Centre.
"Publishing pay ratios will force boards to acknowledge these gaps," he added.
"We look forward to working with business and government to make this new disclosure requirement work as effectively as possible."
The average pay of the chief executives had dropped by a fifth through 2016 to £4.5 million (EUR5.1 million, $6.1 million), according to the latest available figures published by the think tanks.
However, that remains 120 times more than an average full-time worker's salary, a slight drop on the ratio of 122-1 of the previous year.
"We need a significant re-think on how and why we reward CEOs, taking into account a much more balanced scorecard of success beyond financial outcomes, looking more widely at the impacts of businesses on all stakeholders from employees to society more broadly, said Peter Cheese, chief executive of the CIPD.
The two groups said they were working together to push for high pay to be addressed as part of a broader review of corporate governance in Britain, including greater transparency on workforce data.