LONDON - Only 17% of workers in British cities had returned to their workplaces by early August, underscoring the challenge for Prime Minister Boris Johnson to steer the country away from its coronavirus shutdown, data published on Thursday showed.

The Centre for Cities said the data, based on mobile phone signals, showed no increase in the footfall of workers going to city centres between late June and the week starting Aug. 3.

Johnson last month encouraged people working from home to get back to their workplaces to help the economy recover from its 20% contraction in the April-June period, the biggest fall among big developed economies.

"The costs of office closure are becoming clearer by the day," Carolyn Fairbairn, director-general of the Confederation of British Industry, an employers group, said.

"Some of our busiest city centres resemble ghost towns, missing the usual bustle of passing trade. This comes at a high price for local businesses, jobs and communities," she wrote in an article for the Daily Mail newspaper.

Separate figures published by Britain's statistics office showed almost one in four businesses in accommodation and food services and arts, and the entertainment and recreation industries rated their risk of insolvency as moderate or severe.

The Office for National Statistics also said footfall in mid-August in high streets, retail parks and shopping centres had increased to around 70% of its level a year earlier.

Vehicle traffic levels on Aug. 24 were just six percentage points lower than in early February, before the pandemic struck.

(Writing by William Schomberg and Marc Jones) ((william.schomberg@thomsonreuters.com; +44 207 542 7778; Reuters Messaging: william.schomberg.reuters.com@reuters.net))