Tesco has lost its second female director in a week, compounding the problems facing chief executive Dave Lewis as he tries to turn around Britain’s biggest retailer.

Judith Nelson, the UK and Ireland people director, is to leave the supermarket group after 22 years, Tesco confirmed in a statement on Friday.

The announcement follows the shock departure of Jill Easterbrook, who was group transformation director and regarded as a key part of Lewis’s senior team.

Tesco shares have fallen to their lowest in 18 years this week as the company battles falling sales and profits ahead of the vital Christmas period. Despite Lewis cutting prices and pledging to improve customer service, Tesco is still losing market share to the discounters Aldi and Lidl, as well as underperforming its bitter rival Sainsbury’s.

The “big four” supermarket chains, Tesco, Asda, Sainsbury’s and Morrisons, are under pressure from a shift in shopping habits that has seen households move away from buying groceries in one weekly shop. Instead, consumers are spending on food and drink across a number of formats, including online, convenience stores and the discounters.

Tesco shares were down 4.65p, or 3%, to 145.90p in Friday afternoon trading. The share price has fallen 41% since April, when it cleared 250p on hopes that Lewis could turnaround the company quickly.

Lewis replaced Philip Clarke as chief executive in September 2014, and within weeks had uncovered an accounting scandal that sparked an investigation by the Serious Fraud Office, which remains unresolved.

Tesco employs 310,000 people in the UK, meaning Nelson is in charge of the largest private workforce in the country.

A Tesco spokesman said: “After 22 years at the company, UK and ROI people director Judith Nelson has decided to develop her career outside of Tesco. Judith has been a core member of the team for many years and we’d like to thank her for her valuable contribution.

“We’re excited that business support director Natasha Adams will be taking over from Judith early next year. Natasha joined Tesco in 1998 and has always had a focus on service, with recent roles including UK operations people director and customer service director.”