A motion passed at the Lib Dems’ annual conference, with leadership backing, said the party should support a “longer-term objective” of EU membership in the future.
The date would be “determined by political circumstances, subject to public assent, market and trade conditions and acceptable negotiated terms”.
The motion also calls for the “closest possible” alignment between the UK and EU towards a customs union, single market and freedom of movement.
And it urges the Government to honour the commitments made in the Brexit Withdrawal Agreement and the Political Declaration.
An amendment calling on the party to support rejoining the EU immediately was rejected by members.
The Lib Dems campaigned in the 2019 general election to stop Brexit without a second referendum.
An internal inquiry into the party’s poor performance later likened the campaign to a “high-speed car crash”.
The party’s Brexit spokeswoman Christine Jardine said following the debate on the motion: “With our country already facing the biggest economic and health crisis in generations, it is unthinkable that the UK would crash out of the transition period without a deal.
“Boris Johnson must start putting the nation first, rather than sacrificing everything and anything in his ideological pursuit of Brexit. This is about stopping the disaster of food shortages, medicine shortages, and more.
“At every step of the Brexit process, Liberal Democrats were clear in our pro-EU position. We’ll never turn our back on our pro-European values because we are so much better off standing together than we could ever be alone.
“That is why our duty now is to put every last ounce of political energy towards stopping a no-deal and opposing Boris Johnson’s efforts to drag the country’s reputation through the mud by breaking international law.”