The economic shock of a Brexit would cause retail sales volumes to fall by 3% next year, a new report has warned.

The Economist Intelligent Unit study claims that a vote to leave the EU there will be a “short and relatively short retail shock” with sales volumes “flattening out” in 2016.

It predicts this will be followed by a “shrinking by around 3% next year”.

By 2020, the report claims, “nominal” retail sales will be 6% lower than if the UK votes to remain in the EU.

The report suggests a Brexit would cause a period of “uncertainty”, which will see consumers “retrench and consolidate income and expenditure as they watch and wait on the outcome of the negotiation”.

Former M&S boss Sir Stuart Rose has led the Britain Stronger in Europe campaign, while Sir Terry Leahy, Justin King, Marc Bolland and Sir Ian Cheshire has also given their backing to the UK staying in the EU.

Boots boss Stefan Pessina told Retail Week last week that a Brexit would bring a ”lot of negative consequences”.

The report adds: “The supply chains and agreements that have evolved on an even playing field with partners in the EU will need to be revisited as the goal posts begin to move.

“Regulatory requirements relating to safety, quality and consumer protection that are currently consistent across member states will diverge. This will force retailers to consider two sets of rules when assessing their offering between the UK and Europe.”

The Economist Intelligence Unit also claims that a Brexit will hit the UK’s involvement in the single digital economy, which has “particular ramifications for the UK’s highly developed e-commerce sector which would enjoy a comparative advantage on an even playing field”.

However the report suggests there will be some “silver linings” for retail.

“A pro-Brexit cabinet will dedicate itself to making an economic success of secession,” it said. “This would see a focus on business and investment friendly policies with incentives that could be of benefit to some retailers, especially those less exposed to partners or operations across the channel.”