Asda, the UK’s third-largest retailer, has announced that chief executive Sean Clarke will step down from his position and leave the company by the end of the year after just 18 months in charge. Asda said Clarke will be replaced from January 2018 by deputy chief executive Roger Burnley, who joined the company in the last 12 months from Sainsbury’s.

Asda said the transition to make Burnley chief executive had always been the plan. “Asda is a great business and we’ve started to realise its potential again. Sean’s focus on simplifying the business has established a firm foundation on which we can build,” said Burnley.

Asda, whose parent company is Walmart, has been under pressure from Sainsbury’s, Lidl and Aldi. The German discounters in particular have been eating into Asda’s market share and the company has lost its traditional place as the UK’s second largest retailer behind Tesco. Asda recorded seven consecutive quarters of sales declines from 2014 through to 2016.