Sir Ed Davey, in a speech in Manchester on Thursday, will call for referendum divisions to be buried after Britain leaves the European Union at the end of the week.
Britain will cease to be a member of the EU at 11pm on Friday January 31, with Brexiteers planning a celebratory rally in Parliament Square to mark the moment.
Acting Lib Dem leader Sir Ed, in an apparent admission that the pro-EU campaign is over following Boris Johnson’s crushing election win, said the country “must no longer be divided by Leave and Remain”.
The Lib Dems took only 11 seats at the general election after standing on a platform of cancelling Brexit if the party won a majority.
The party had gone into the December poll with 20 MPs, having been boosted by a number of defections during the last parliament, but did not manage to hold on to its gains or its leader.
Speaking on the UK’s penultimate day as a member of the Brussels bloc, Sir Ed is expected to say: “Tomorrow will be a celebration for some. But for others it will be a dark day.
“For the millions of us who marched against Brexit and the millions who voted to stay, tomorrow will be desperately hard.
“We built the largest pro-European movement this country has ever seen.
“And though ultimately we did not succeed in stopping Brexit, I am immensely proud of all that we have achieved.”
Addressing the need for unity after the UK’s withdrawal, he will add: “We must no longer be a country that is divided by Leave and Remain, but that means we must heal our country’s other divides too.”
Sir Ed is standing in as co-leader of the party, along with Baroness Brinton, after Jo Swinson lost her East Dunbartonshire seat at the election to the SNP.
The party is not due to elect her successor, who will come from its pool of 11 MPs, until July 15.
Former energy secretary Sir Ed will use his speech to warn that there remain “serious divisions to fix” in the country, including the rise of English and Scottish nationalism.
“If Brexit has taught us anything, it is that there are many serious divisions to fix,” he is preparing to tell supporters.
“The UK is divided by inequality of opportunity, wealth, power (and) hope.
“And the forces of English and Scottish nationalism unleashed by these divisions, cannot bring our fractured country together – by their very nature they seek to divide our United Kingdom further.”
He will say it is up to “progressive parties” to “fight to disperse power and wealth across our country”.