Asda’s current president and chief executive, Sean Clarke, will leave his post at the end of the year after just 18 months in the job.He will be replaced by current deputy chief executive Roger Burnley, who joined Asda a year ago from rival supermarket Sainsbury’s.Asda said Mr Clarke, who used to work for parent company Walmart, would “remain engaged” with the US giant.Asda said it was always its plan for Mr Burnley to take over as boss of Asda.Dave Cheesewright, chief executive of Walmart International, said: “Roger was purposefully brought back to Asda to partner with Sean ahead of the transition to Roger taking up the position of CEO.”He and Sean have worked as a great team and I’m really confident in Roger’s ability to continue building upon our returning momentum.”Fierce competitionMr Burnley is familiar with Asda, having worked at the UK’s number four supermarket between 1996 and 2002. Asda is caught up in a fierce battle for market share as the UK’s grocers try to maintain profits amid rising prices and falling incomes.On top of that, the UK’s big-name supermarkets are under attack from fast-growing discounters Aldi and Lidl, where sales are growing several times faster.Some, including Asda, are cutting costs. Last month, Asda said it was shedding almost 300 jobs at its head office. Asda’s performance has improved recently. It announced its first quarterly sales growth of 1.8% in three years this August.