Staff were informed of Forrest's move earlier today, before he goes on gardening leave until the new year.
A spokesperson for Premier Inn said: "We can confirm that John Forrest is leaving the business this month to take up a senior role externally.
"Simon Ewins, who was previously the operations director for Whitbread Restaurants over a three-year period and is currently the chief operating officer for Hub by Premier Inn and CSR, will become chief operating officer on an interim basis.
"We'd like to thank John for his commitment and contribution to the business over the last nine years."
Forrest began his career at Whitbread in 2007 as business development director for Whitbread Hotels and Restaurants to lead the growth programme for Premier Inn. It is a role he has achieved with great success, with the brand opening its 700th hotel this week in Kingston-upon-Thames, Surrey.
He was promoted to brand operations director for the Table Table restaurant brand in 2010, before joining the Whitbread management board in March 2011 when he became chief operating officer for Premier Inn.
Described by a former employee of Whitbread as "the heart and soul of Premier Inn", Forrest has been totally focused on ensuring every new opening will live up to the brand's pledge of 'a good night's sleep guaranteed', with a guest who does not sleep well entitled to claim a full refund.
To ensure the guest enjoys the best possible experience, Forrest has stayed at every newly launched hotel - opening at an average of about one every 10 days - to carry out a thorough audit of the property.
Forrest began his career at Marks and Spencer before moving to senior operational roles at The Body Shop and The Co-operative Group.
In Whitbread's interim trading statement for the first quarter of the financial year - published earlier this month - the company reported that Premier Inn was its strongest performing sector with like-for-like sales up by 6.3%, compared with total like-for-like sales of 4.3%. The brand currently has 59,000 bedrooms and aims to have a total of 85,000 open by 2020.