Exporters to the UK have been shrinking the size of products such as chocolate bars and fruit juices since the Brexit vote but not reducing the prices they charge British consumers in the shops, the government has been privately warned by European port officials.

Minutes of a meeting between the Department for Transport and ports on either side of the Channel reveal concerns that the drop in the value of the pound since last June has prompted companies to offer shoppers in the UK less value for money for food and drink.

The issue was raised by officials speaking for Zeebrugge, the Belgian port through which 17m tonnes of products arrive in the UK every year, the Guardian can reveal. The minutes of the meeting in Brussels note: “The Port of Zeebrugge expressed its concerns over the currency devaluation of the pound sterling, which since the referendum on Brexit fell 15%.”

They continued: “The industry that is located in the port of Zeebrugge has responded to this fall of the pound sterling by offering slightly smaller units of certain products for export to the UK, yet keeping the same selling price for those products (examples include bottling orange juice in bottles of 950ml instead of one litre, decreasing the amount of chocolate in a chocolate bar, etc).

“In short, customers pay the same price and have the same quality of product, but receive less quantity.”

The UK imports 48% of its food and the plummeting value of the pound has increased costs for importers. Companies have until now, however, largely denied the influence of Brexit on cuts to the sizes of products, including the infamously redesigned Toblerone bar.

As Theresa May took to the campaign trail championing the “great opportunities of Brexit,” Tim Farron, the leader of the Liberal Democrats, warned: “This is a stealth Brexit squeeze on British consumers. While Theresa May is setting her rosy vision of Brexit, the reality is shoppers are already losing out.

“We will stand up against a bad Brexit deal that would push up prices further, and give people the final say.”

The shrinking of chocolate and drinks being sold in the UK was, however, only one of the issues raised during the discussions between Whitehall officials and the European Sea Ports Organisation’s Brexit working group on 31 January, the document reveals.

Dover, Calais and Dublin ports voiced their fears about how they would cope with installing the infrastructure required for customs controls once the UK leaves the customs union and single market in March 2019.

Calais said it “does not have enough available space to accommodate customs controls for the vast amount of traffic passing through the port,” the minutes report. “The port was not planned for accommodating huge queues of lorries.”

The minutes add: “Calais also highlighted that currently there is not enough customs personnel to perform the customs controls if the UK would leave the customs union” and “the UK’s intention to constrain the free movement of people is considered as problematic”.

In this scenario, the port explained, “the UK would have to conclude bilateral visa agreements with third countries, which would complicate the visa controls for the French police”.

The port’s representatives added that “given the massive amount of passenger traffic that runs through the port, the French police would not be able to deal with all the exceptions”.

The British transport officials present were also told that “if customs controls would be reintroduced, the port of Dublin would need an estimated three hectares of land dedicated for customs”.

The minutes note: “This would be problematic, as the port already has space constraints to expand due to its location. The port’s approach in its masterplan is to further develop the land it has now, instead of expanding.

“Thus, dedicating three hectares of the available land to customs would be problematic. Reintroducing customs controls would also come with additional costs for customers of the port, since the turnaround time for a ship would increase. A possible solution to cope with the aforementioned customs related challenges could be to have UK customs in the port of Dublin and Irish customs on UK soil.”

A spokesman for the Department for Transport said they did not comment on leaks.