Workers walk under the quay cranes at DP World London Gateway Port in Stanford-le-Hope, Essex, England | Dan Kitwood/Getty Images UK risks delays at ports if it fails to strike customs deal: report Trade association said delays could cause ‘gaps on shelves.’

Goods coming into Britain could be stuck at ports and other points of entry for up to three days if Theresa May fails to strike a proper customs agreement with the EU, according to a report published Wednesday by the British Retail Consortium.

The trade association said such a delay could risk "gaps on shelves."

Annual customs declarations in the U.K. are expected to rise from 55 million to 255 million after March 2019, when Britain leaves the EU, the BRC said, based on estimates from the government's customs department.

If there's no system of controls after Brexit that ensures products are imported without delays or additional costs, waiting times of up to two or three days are likely, it said after carrying out research alongside the International Meat Traders Association.

The BRC added that there is a need for “significant investment” in the U.K.’s ports, roads and infrastructure to ensure that they are ready for the post-Brexit period.

"Whilst the government has acknowledged the need to avoid a cliff-edge after Brexit day, a customs union in itself won’t solve the problem of delays at ports," Helen Dickinson, chief executive of the BRC, said.

"To ensure supply chains are not disrupted and goods continue to reach the shelves, agreements on security, transit, haulage, drivers, VAT and other checks will be required to get systems ready for March 2019."

The latest round of Brexit talks got underway in Brussels on Monday.

Correction: This article has been updated to correct the number of customs declarations.