Britain’s retailers have recorded the longest period of falling sales for almost eight years, fuelling mounting concern for the economy ahead of Brexit.

The Confederation of British Industry said retail sales across the country had dropped for a third month running in July, marking the longest period for sliding sales since 2011.

Against a backdrop of heightened political uncertainty with less than 100 days to go before Brexit, sales dropped most for department stores, clothes shops and retailers selling other consumer goods such as jewellery and flowers.

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The survey of 93 leading high street firms and retail wholesalers found that sales volumes fell in the year to July, albeit by a lesser extent than in June. Retail firms placed fewer orders with their suppliers for the third consecutive month, and a further decline is expected in August.

Rain Newton-Smith, chief economist at the CBI, said the figures were hugely concerning. “The sun is clearly not shining on the British high street,” she said. “The UK economy has reached a fork in the road. The new prime minister must now do everything in his power to achieve a good Brexit deal, thus protecting jobs and our economy.”

The warning shot from the retail industry comes as a no-deal Brexit becomes an increasingly likely prospect following the elevation of Boris Johnson to prime minister. Economists warn that household budgets could come under intense pressure under such a scenario, should living costs and unemployment rise.

The business lobby group’s monthly retail sales index rose to -16% in July from -42% in June. Although a slight improvement on the month, the balance remains in negative territory, showing that more retailers reported a slump in sales than a rise. City economists had expected a reading of -10%.

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Internet sales growth picked up on the previous month, although the CBI warned the volume of goods being bought online remained below the long-term trend. It said the growth in sales was also expected to slow further in the year to August.

Economists have sounded a note of caution that the CBI retail sales monitor is often proven too pessimistic. The most recent official figures for the high street show that sales have been resilient in recent months.

Suggesting that consumers are continuing to take the heightened levels of political turmoil over Brexit in their stride, the Office for National Statistics said the volume of goods sold in May increased by 1% on the month. Sales were up on the year by 3.8%, beating the forecasts of City economists.