The High Pay Centre think tank said chief executives in the FTSE 100 were paid an average of almost £4.3 million in 2012, equivalent to an hourly wage of well over £1,000, compared with the country's average annual wage of £26,000.
Executive pay has increased by 74% over the past decade, while wages for ordinary workers have remained "flat", the study found.
Company bosses who returned to work on Monday after the festive break will have made more money in 2014 by mid-morning today than the annual salary of average workers, said the report.
High Pay Centre director Deborah Hargreaves said: "Fat cat Wednesday highlights how insensitive big company executives have become.
"When top bosses take home more in two-and-a-half days than the average worker earns in a year, there is clearly something wrong with the way pay is set for both bosses and workers."
TUC general secretary Frances O'Grady commented: "Soaring pay inequality, with top bosses now taking home more in a few days than most workers earn in a year, is damaging our economy.
"Workers need better pay rises so that the recovery is built on growing incomes, rather than falling savings and mounting household debts. But Britain's fat cat bosses are hoarding earnings owed to staff for shareholders and themselves.
"That's why we need workers on remuneration committees to knock some sense into top bosses' pay."
Katja Hall, CBI Chief Policy Director, said: "High pay should always be associated with high performance.
"Employers recognise that staff have seen a real squeeze in earnings which was the trade off that enabled more people to keep their jobs during the recession. The good news is that as growth picks up, so too will pay settlements."