It is not known where Cook, who has been head of MWB's 26-strong boutique hotel division since 2004, will move to, but he told BigHospitality's sister publication M&C Report that he had 'one or two options' for his next move and confirmed that he would be staying within the hospitality industry.
Cook, who has helped drive the growth of Malmaison and Hotel du Vin over the past eight years and introduced a number of new concepts - most recently the the standalone restaurant concept Smoak within Malmaison, said it was the right time to leave the business.
"I think the time is now right to move on to something new while bringing in a fresh pair of eyes and new ideas will help to develop the business further as well as refresh the brands," he said in a statement.
"It has been a truly exciting and rewarding journey over the past eight years. I believe I am leaving a business that is in great financial shape, has a fantastic team of people and two brilliant brands."
Richard Balfour-Lynn, chief executive of MWB, said the company understood Cook's desire to 'seek new challenges'. "Equally it is the right time for a successor to ensure the brands maintain their cutting edge approach to hospitality and to lead Malmaison's entry into the international market," he said.
"Over an eight year period Robert, working closely with MWB's executive management team, has helped drive the growth of the Malmaison and Hotel du Vin brands throughout the UK and together we have developed the business into one of the country's foremost boutique hotel groups."
Earlier this month MWB refinanced a £285.5m debt facility against its Malmaison and Hotel du Vin business which, together with the £100m net proceeds from the sales and leasebacks of five of its Malmaison hotels, enabled it to reduce the hotel company's debt to £180m.
Balfour-Lynn said: "The successful refinancing of the Malmaison Group, combined with an established and strong operating team within the brands, provides a great platform for growth in the future and we are optimistic over the medium-term for the success of our business."