Scottish fishermen and seafood exporters rallied in central London on Monday, parking slogan-bearing lorries near the UK parliament in protest at post-Brexit rules they say are threatening the industry's future.

The protesters descended on the capital after facing weeks of chronic delays to shipments of fresh and live produce, prompted by Britain's departure this month from the European Union single market and customs union.

Requirements for exporters to complete reams of new paperwork when sending seafood to Europe have rattled a sector that relies on rapid delivery schedules.

"What they're asking us to do... just isn't possible with live shellfish," Allan Miller, who runs one such business in Scotland, told AFP, adding the process needed to be "streamlined".

He noted the "mountains of paperwork" -- from customs declarations to health certificates -- and "astronomical" related costs were crippling his and other operations.

"We always knew there was going to be problems but... never did we foresee what we've got in front of us just now," Miller said.

Prime Minister Boris Johnson reiterated Monday that the dire situation they describe is due to "teething problems" exacerbated by the coronavirus pandemic.

"Unfortunately, the demand in restaurants on the continent for UK fish has not been what it was before the pandemic," he said in response to the protests.

However, it is unclear the problems encountered by the sector are down to depressed demand from Europe.

Johnson has promised compensation for those financially impacted by the new bureaucracy, while insisting his post-Brexit trade deal with the EU offers them "great opportunities" in the future.

But at Monday's protest, trucks were clad with signs reading "incompetent government destroying shellfish industry" and "Brexit carnage".

Miller said the new red tape needed resolving immediately or the industry faced ruin.

"There was always going to be teething problems but every single business can't go on the way it's going just now," he added.

Britain formally left the EU last January but remained within its regulatory orbit until the end of the year under the terms of its divorce.

Although the two sides struck a Christmas Eve free-trade deal, avoiding trade tariffs and quotas, the UK's departure from the EU single market and customs union has resulted in bureaucracy and hold-ups.

Instead of being shipped straight to market in northern France, and then to tables across Europe, pricey seafood shipments face going straight into landfills.

Some Scottish fishermen are choosing to keep their boats in port rather than risk their catch being caught up in the delays or see it go to waste.

"Many fishing vessels are tied to the quay wall," the chief executive of the Scottish Fishermen's Federation, Elspeth Macdonald, said in a letter to Johnson last Friday.

"Of the others that can go to sea, some are now making a 72-hour round trip to land fish in Denmark, as the only way to guarantee that their catch will make a fair price and actually find its way to market while still fresh enough to meet customer demands."

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