High street retailers must give each shopper a minimum of 10 sq metres of space if they are to maintain social distancing rules, a new study has found, prompting businesses to complain that a lack of official guidance has left reopening plans in chaos.

With 10 days to go until most non-essential shops reopen in England, a research paper by Manchester Metropolitan and Cardiff universities has concluded that larger shops must allow substantially more than a 2-metre distance for each person so that they can move freely.

The study, which uses tessellated modelling - a combination of circles and squares to indicate available space – finds that people need 10 sq metres in smaller shops, 11 sq metres in larger shops, and 12 sq metres in outdoor spaces where they move more rapidly. That is up to three times as much space as the 4 sq metres needed around stationary people to maintain 2 metres of social distancing, according to the modelling.

The expert analysis was produced because its authors say that the official advice on reopening fails to set out how to practically enforce the 2-metre distancing guidelines.

Retailers say the report highlights the failure of the government to produce adequate advice for shops as they return to business. The official advice simply tells people to consider how many people can “reasonably” fit and to “limit the number of customers in the store”.

“I’m surprised we haven’t got clear guidance that says how many people you can have in a shop of a certain size,” said Andrew Goodacre, chief executive of the British Independent Retailers Association (BIRA). “That would create an equal footing in every shop. The risk is that you get a race to the bottom on a street. Consumers will be wary of shops that have no hard and fast policies.”

Large and small shops are scrambling to prepare for the reopening of the high street on 15 June, with businesses experimenting with measures from “browsing by appointment” to using floral designs for distance warning stickers because they are believed to be more reassuring.

But Goodacre said the difficulty of estimating how many shoppers would be in an average store – alongside complicating factors such as the fact that customers were more likely to buy something if they had been queuing – made it hard to predict the impact of social distancing on business.

With an estimated 70% of pre-coronavirus trade needed to break even, Goodacre said retailers would “be lucky to see 30-40% of sales with this kind of capacity”.

The report’s conclusions are based on the speed and unpredictability of people’s movements, which means they are much more likely to get near each other when shopping than when queuing or sitting at a desk – and therefore require a bigger “halo”.

Chris Turner, the chief executive of British BIDs – the representative group for Business Improvement Districts – said the report showed that maintaining a 2-metre distance was “hugely problematic”.

“When you do the sums, you find that a small cocktail bar can fit about two people in,” he added. He warned that more planning needed to be done to prepare shared high street space, saying: “Retailers are finding creative ways to deal with this 2-metre malarkey – but they can’t afford to have them all queuing outside at the same time as it stands because if everybody does that it’ll be chaos.”

Prof Cathy Parker, one of the report’s authors, said that “people don’t want to go from one place to another and see that the numbers are really different. Anything that’s going to make people think the rules are being broken and it’s a risky environment, that’s a problem.”

“It’s fine to see everybody queuing up, that doesn’t look like a risk,” she added. “But telling people to keep 2 metres apart in shops where people are bunching … You can tell retailers to manage capacity but how do they work that out? None of it is visible in the government guidelines.”

Ministers are continuing to debate whether social distancing rules could be relaxed to 1 metre without risking a rapid rise in the coronavirus “R” rate that will determine whether the UK faces a second wave of infections.

Some shops, pubs and restaurants are calling for a change to 1-metre distancing, in line with World Health Organization guidelines. The research found that such a reduction would mean that only about 5 to 6 sq metres would be needed per shopper.

But while Turner said that for many hospitality businesses “it will not be financially viable to reopen unless the distance is reduced from 2 metres” and called for the minimum to be reviewed, he noted that the uncertainty over the final guidance risked “tossing all the planning into disarray”.

“There’s a real sense that the government is making it up as it goes along,” he said.

​• This article was amended on 9 June 2020 to clarify the type of modelling used in the study, and to remove a graphic which, ​while showing the correct figures, did not show the exact modelling used to arrive at these figures.