Boris Johnson has been urged to negotiate an adjustment period into any final trade agreement with the European Union to provide “breathing space” of up to six months for businesses.

The post-Brexit trade talks have entered into a decisive week, with Cabinet ministers among those signalling that the discussions have entered into last chance saloon territory.

With Thursday touted as the latest deadline for negotiations, the Liberal Democrats and a leading business figure have joined forces to press the Prime Minister to effectively extend the transition period to give businesses more time to adjust to any new regulations.

The transition period is due to end on December 31, meaning a new customs regime and other new Brexit-related rules are due to come into force at the start of next year.

But Sir Ed Davey, leader of the Lib Dems, and former Siemens boss Juergen Maier have told Mr Johnson in a letter, seen by PA news agency, that “breathing space” is required for businesses to adjust to any changes agreed with Brussels.

They warned that industry is already dealing with the “unprecedented” challenges caused by Covid-19 and that the threat to some firms was “existential”.

“Uncertainty is bad for business at any time, but the combination of the challenges of battling the impact of coronavirus restrictions alongside having now to cope with the huge challenge of complying with new trading procedures, at very short notice, could be very damaging,” they said.

“Indeed, some in the business community tell us they are seriously concerned that the threat they face due to these twin challenges is existential.

By phasing in new rules, regulations and procedures, our businesses would have the breathing space they need to prepare and adapt

“Industry bodies from every sector of the economy have warned that they will need time to adjust.

“So we urge you to negotiate the reasonable and practical measure of a three to six-month adjustment period in the EU trade deal, to save jobs and businesses.

“By phasing in new rules, regulations and procedures, our businesses would have the breathing space they need to prepare and adapt. The UK will also have time to prepare for the logistical burden of customs checks at the border.”

The Lib Dems pointed to comments made by Environment Secretary George Eustice on Sunday in which he suggested there would be a “phasing-in period” for checks on goods entering Northern Ireland at the end of the Brexit transition period, and urged ministers to widen the policy to cover all sectors.

The call comes as the UK’s chief negotiator Lord Frost and his EU counterpart Michel Barnier continue negotiations in Brussels with just over six weeks to go until the end of the transition period.

The talks, which follow a similar round in London last week, come ahead of a European Council video summit on Thursday which has been touted as a deadline for a draft deal.