A model walks the runway for the Mary Katrantzou collection during 2019 Fall/Winter London Fashion Week on February 27, 2019. Photo: IC

Britain's fashion industry is "at real risk of decimation" by the post-Brexit trade deal with the European Union and current government policy, a leading forum warned on Monday.

More than 400 industry figures signed an open letter from the Fashion Roundtable to Prime Minister Boris Johnson arguing there was a "gaping hole" in his deal where free movement of goods and services for creatives should have been.

They said the agreement, finalized on Christmas Eve just a week before the UK left the bloc's single market and customs union, had failed to deliver on promises of frictionless future trade.

Their concerns echo those of other sectors, from music to fisheries, who argue the new bureaucratic regime is endangering their future.

"We have been disregarded in this deal and our concerns overlooked in current policy decisions," their letter said.

"We call on the government to stand by their commitment to frictionless work travel for British creatives, including the fashion and textile industry and their equipment."

The open letter, signed by models such as Twiggy and designers including Katharine Hamnett as well as bosses of fashion retailers, urged Johnson to take specific steps to address their concerns.

They include adding more reciprocal elements to the agreement in areas such as VAT, making it easier for garment workers to come to Britain and for the government to offer more industry support.

"Ours is a thriving industry, based on global leadership, complex supply chains and above all a deeply interconnected relationship with our overseas colleagues," it stated.

The letter added the sector contributes 35 billion pounds ($48 billion) to UK GDP and employs almost 1 million people, but was "at real risk of decimation by the Brexit trade deal."

Britain formally left the EU a year ago but remained bound by its rules and regulations until 2021.

The free trade agreement struck by Johnson and Brussels after months of strained negotiations saw no tariffs or quotas imposed.

However, after the UK chose to leave the single market and customs union, it was unable to prevent reams of new red tape for British businesses trading with Europe.