Boris Johnson has warned Emmanuel Macron that Brexit negotiators will need to bridge "significant gaps" within days, as Tory sources suggested the French president's stance on fishing was standing in the way of a deal.
In a telephone call on Saturday morning, the Prime Minister warned Mr Macron that "progress must be made" on fishing quotas and the EU's demands for the UK to abide by a "level playing field" of rules, including on industrial subsidies.
Lord Frost, Mr Johnson's chief negotiator, is in the middle of a fortnight of intensive talks with his EU counterpart Michel Barnier as they attempt to avoid a no-deal outcome at the end of the transition period on December 31.
Mr Johnson's intervention came as Conservative sources said the prospect of a deal now turned on whether Mr Macron was prepared to make a "political decision" to compromise on fish - a totemic issue for British and French coastal towns and villages.
The EU's position until now has been to insist on replicating the Common Fisheries Policy, maintaining the access and quota shares enjoyed by EU fishermen before Brexit.
But UK negotiators are insisting that British waters should be subject to annual negotiations that allow British fishermen to reclaim a much greater share of fish.
In an attempt to break the deadlock, the UK has proposed a three-year transition period for fisheries, during which the catches of EU fishermen would be gradually reduced before the implementation of new quotas.
But, in a summary of the call with Mr Macron, a Downing Street spokesman said: "The Prime Minister emphasised that progress must be made in the coming days to bridge the significant gaps, notably in the areas of fisheries and the 'level playing field'."
Separately, Alok Sharma, the Business Secretary, is writing to more than 600,000 firms, urging them to prepare themselves for the end of the transition period.
Mr Sharma said: "When the transition period ends on 31 December 2020, there will be a guaranteed set of changes and opportunities for which businesses need to prepare as the UK leaves the customs union and single market, including changes to the way businesses import and export goods, the process for hiring people from the EU, and the way businesses provide services in EU markets."
Mr Sharma's letter comes after Alex Chisholm, the Cabinet Office permanent secretary, warned that many businesses still believed the transition period would be extended.