Britain's five biggest business lobby groups are calling for an urgent Brexit transition deal, or they warn the UK risks losing jobs and investment.
In a joint letter being sent to Brexit Secretary David Davis, the groups, including the Institute of Directors and CBI, will say time is running out.
Sources told the BBC the letter is still in draft form, but will be sent in the next day or two.
A government spokesman said the talks were "making real, tangible progress".
The other lobby groups backing the letter are the British Chambers of Commerce, the Federation of Small Businesses, and the EEF manufacturing body.
Together they represent companies employing millions of workers.
There has been a growing anxiety among businesses at what they see as a lack of progress in the Brexit negotiations.
One of the five groups told the BBC it was felt a joint letter would "emphasise our wish for a deal and clarity".
They say it is important that the Brexit transition period matches as closely as possible current trading arrangements with the EU.
Theresa May has suggested a period of about two years, with the UK and EU trading on broadly similar terms to now and payments to Brussels to meet Britain's budget commitments.
But although EU negotiators have agreed to start preliminary work on a future relationship, they still want more concessions on the UK's so-called "divorce payment" before starting talks on trade and transition.
Sky News and the Guardian reported they had seen the draft letter, which says an agreement on a transition "is needed as soon as possible, as companies are preparing to make serious decisions at the start of 2018, which will have consequences for jobs and investment in the UK".
The letter reportedly adds: "It is vital that companies only have to undertake one adjustment as a result of the UK's withdrawal, not two - and that businesses, the UK government and authorities in the EU have enough time to make the changes needed to deliver Brexit successfully."
The BBC understands that the business groups want the contents of the letter to remain private.
Concern about the loss of UK jobs and investment was underlined last week when the boss of investment banking giant Goldman Sachs, Lloyd Blankfein, tweeted that he will be "spending a lot more time" in Frankfurt.
Goldman, which employs about 6,000 people in London, is building up its presence in the German financial city ahead of Brexit.
Earlier this month, the deputy governor of the Bank of England, Sam Woods, warned that the UK and the EU must agree a transition deal by Christmas or companies would start triggering contingency plans.
And in a survey released on Monday, the EEF said that Brexit uncertainty was holding back the plans of manufacturing firms to invest in new plant and machinery.
The EEF said the outlook for investment among its members was "finely balanced", with 51% intending to spend more in the next two years. For the rest, uncertainty over the UK's exit from the EU was holding back planned spending.
Mr Davis is due to travel to Paris for Brexit talks on Monday after France appeared to emerge as the most hardline EU member state when it comes to the divorce bill.
The prime minister is also due to update the Commons on Monday on the progress made during last week's summit of EU leaders in Brussels.
It is thought that Mrs May will say that negotiations are "deeply technical", but she has not forgotten that the lives of millions of people are at the heart of the process.
A spokesman for the Department for Exiting the European Union said the prime minister proposed a strictly time-limited implementation period in her Florence speech and had been clear that agreeing this principle early in the process would minimise unnecessary disruption to businesses.
He said: "We are making real and tangible progress in a number of vital areas in negotiations. However, many of the issues that remain are linked to the discussions we need to have on our future relationship.
"That is why we are pleased that the EU has now agreed to start internal preparatory discussions on the framework for transitional arrangements as well as our future partnership."