They also say that nothing can mitigate the losses Britain would suffer in the event of a ‘no-deal’ outcome with the EU.
Such a scenario would see tariffs and other impediments imposed on trade between the UK and the EU.
Though both sides would suffer from the new barriers to trade, most economists think Britain would be hit disproportionately.
In 2019, the UK exported some £36.7 billion of goods to Germany, Europe's largest economy, or 10 percent of its total. Exports to Japan were just £7.2 billion, or 1.9 percent of the total.
The talks with the EU have not collapsed yet and discussions are set to resume on Monday in Brussels.
Though the UK left the bloc on January 31, it is in a transition period that effectively sees it benefit from tariff-free trade until the end of this year.
The discussions are about agreeing on the broad outlines of the trading relationship from the start of 2021.
Concerns over a post-Brexit trade deal have heightened in the past few days since the British government said that new legislation breaches parts of the withdrawal agreement, which allowed for the country’s smooth departure from the bloc.
The diplomatic shockwaves from the British announcement could derail any hopes Prime Minister Boris Johnson may have of negotiating a US trade deal.