The Financial Reporting Council's revised corporate code came into effect in January, requiring public companies in Britain to explain if a board chair has remain unchanged for more than nine years.
Under the code, where the chair was previously a non-executive director, as was the case for Williamson, time served in that capacity counts towards the nine year period.
Williamson joined Imperial's board in 2007, was named senior independent non-executive director in February 2012 and then appointed deputy chairman in January 2013. He became chairman of the company in February 2014.
Shares in Imperial, which makes Gauloises and Winston cigarettes and is one of the world's biggest tobacco companies, had fallen nearly 15 percent from August through Friday. They were up 1.6 percent at 2,579 pence at 1547 GMT.
At its annual general meeting in Bristol last week, Williamson's re-election was opposed by nearly 18 percent of the votes cast by Imperial shareholders.
Much of the opposition was thought to be due to fears of "overboarding," or taking on too many other boardroom posts, the Times, which first reported Williamson's planned exit, said.
The revised FRC code says companies should explain why they allow non-executive directors such as Williamson to take on multiple appointments.
In addition to his responsibilities at Imperial, Williamson is also chairman of engineering company Spectris and a board member at National Grid.
(Reporting by Martinne Geller and Huw Jones in London and Shashwat Awasthi and Tanishaa Nadkar in Bengaluru; Editing by Sai Sachin Ravikumar and Alexander Smith)
