Charity the Equality Trust said the salary gap between bosses and their employees is fuelling a "damagingly high" level of inequality across the UK.
The median annual pay of FTSE 100 CEOs in 2019 was £3.6m (around £941 per hour of their 12-hour days), according to the High Pay Centre.
Assuming they start work at 8am, they had already earned a Preston full-time worker's median salary (£26,938 in 2020) by around 1pm on Wednesday, January 6 – just the third working day of the year.
This means it would take an average Preston employee 134 years to earn the annual salary of a top CEO.
The High Pay Centre said CEO pay levels did not change much from the year before, while the average salary in Preston was down from £27,324 in 2019.
The median is used to stop figures being skewed by particularly small or large wages.
The think tank warned it was still too early to take the impacts of the coronavirus on pay packets into account, but estimated CEOs would have to work just 33 hours this year to surpass the average UK salary, of £31,461.
High Pay Centre director Luke Hildyard said: "Factors such as the increasing role played by the finance industry in the economy, the outsourcing of low-paid work and the decline of trade union membership have widened the gaps between those at the top and everybody else over recent decades."
He added that these figures will raise concern over whether major companies are distributing pay fairly, and should prompt debate about the effects of inequality.
Dr Wanda Wyporska, executive director of The Equality Trust, said: "This huge gap between the pay packets of bosses and their employees is not just highly unfair, but contributes to the damagingly high level of inequality we see in the UK.
"Crucially we see the results in our everyday lives because in countries with high levels of inequality, we also see high levels of poor mental and physical health, violence, infant deaths, obesity and lower levels of social mobility and educational attainment."
With women in Preston earning less full-time on average than men (£25,493 compared to £28,025), FTSE 100 bosses will surpass their annual wage in just 27 hours.
The Investment Association, a trade body that represents investment managers, said: "Investors expect companies to treat their executive directors and workforce consistently when it comes to pay and not to isolate executives from the impact of Covid-19."
Director Andrew Ninian added: "Our members are calling on companies to ensure CEO pay is proportionate and aligned with performance, so that it reflects the experiences of employees, shareholders and other stakeholders."