European business leaders have told Theresa May that progress is urgently needed in Brexit talks or UK jobs and investment could be lost.
The Prime Minister hosted senior industry figures from across the continent in Downing Street to hear their concerns.
Emma Marcegaglia, president of the lobbying group BusinessEurope, said it was crucial for progress to be made within two weeks in order for trade talks to be given the green light by EU leaders at December's summit.
She told reporters in Downing Street: "The message is that as businesses all over Europe we are strongly united.
"We are very concerned for the fact that we don't really see progress in the negotiation and we know that there are two weeks to get to sufficient progress so that in the council in December it can be decided to go on to phase two.
"So, these two weeks are absolutely important.
"We need to have a transitional period where the UK stays in the customs union and single market because companies need certainty."
She added: "We don't want uncertainty, we are very concerned. We know that if companies don't see certainty probably they will have a contingency plan and probably they will leave the UK, or they will invest less.
"So, these two weeks are extremely important."
The Confederation of British Industry (CBI) as well as organisations from Germany, France and other European Union nations were present at the talks with Mrs May, Business Secretary Greg Clark and Brexit Secretary David Davis.
Ms Marcegaglia said both the UK and Brussels had to work to secure a deal.
"My view is that they both have to work more," she said, but because it was the UK's decision to leave it was for Mrs May to put a "real, concrete proposal" on the table.
CBI director-general Carolyn Fairbairn said the message from European business was that "we need urgent progress on the divorce issues so that we can move on to trade and transition by December".
Firms are "beginning to press the button" to implement contingency plans in preparation for a no deal Brexit, she said, but progress in the negotiations could result in a "win-win" situation.
She told the Press Association: "The overall impact, undoubtedly, of the ongoing uncertainty is fewer jobs, less investment, less economic prosperity across Europe.
"That was the mutual interest that was dominating today in the meeting, the idea that there is a win-win out here - sometimes it feels like lose-lose, we can turn this round into win-win and the next few weeks will be absolutely critical to that."
The EU's chief negotiator Michel Barnier indicated on Friday that progress is needed within two weeks on so-called divorce issues - including the financial settlement - for the bloc's leaders to consider moving onto the next stage of negotiations which would look at a trade deal and transitional arrangements.
The Government's Brexit legislation returns to the Commons tomorrow, with Mrs May facing a potentially bruising battle as she tries to avoid rebellions by both the Eurosceptic and pro-EU wings of her party.
Ahead of the parliamentary battle, Labour's shadow Brexit secretary Sir Keir Starmer said leaving the EU without a deal could bring down the Government and appealed to Tory rebels to back efforts to water down one of Mrs May's negotiating red lines.
Sir Keir told BBC Radio 4's Today programme: "No deal is a very, very bad outcome.
"Taken literally, it means we have not agreed anything, and that means we haven't agreed anything about EU citizens, we haven't agreed anything about the border in Northern Ireland, we haven't agreed anything on security.
"I think that sort of no deal is unthinkable.
"In those circumstances I think the Government would have to seriously consider whether it could continue."
As the EU (Withdrawal) Bill goes into committee stage, Sir Keir said he hoped for support from Tory rebels for a Labour amendment allowing the European Court of Justice (ECJ) to maintain its role during a transition period.
Sir Keir said a future relationship with the ECJ after transition would be a "matter for negotiation".
Meanwhile, a Government amendment to the Bill formally ending Britain's membership of the EU at 11pm GMT on March 29 2019 was strongly condemned by Conservative former attorney general and prominent Remain campaigner Dominic Grieve.
"I think it is an incoherent and thoroughly stupid amendment and it won't have my support," he told Sky News.