Theresa May will face a Commons battle if she tries to force a hard Brexit on the UK, with Labour warning it will not give the Prime Minister a "blank cheque" to trigger Article 50.
If the Supreme Court upholds the ruling that MPs must be given a say before the Prime Minister can formally start the Brexit process, Labour will seek to amend the legislation unless Mrs May produces a "meaningful plan".
In a major speech, shadow Brexit secretary Sir Keir Starmer will vow to fight any strategy that "tears us apart from our EU partners", instead backing an approach to build a "new and strong relationship" with the other 27 countries.
Sir Keir will use his speech in London to set out his goal of making jobs, the economy and living standards Labour's priority in the "battle of our time".
He will accuse the Prime Minister of failing to unite a deeply split country, saying that instead of trying to "repair the breach" following the EU referendum she has "only had a message for one side of the divide".
Sir Keir will paint a hard Brexit as "a future that tears us apart from our EU partners", outside the single market and customs union.
It would mean "reverting to World Trade Organisation rules which would entail a range of harmful new barriers to trade and a desperate rush to sign new agreements with third party states to compensate".
"A global race to the bottom which would not only put our economy and jobs at risk, but which would also abandon our shared scientific, educational and cultural endeavours with the EU."
Sir Keir will argue for a Brexit model that "does not damage our ability to trade goods and deliver services with and to the EU".
Setting out his alternative to hard Brexit, he will say: "The second version of our future is a version where we exit the EU but build a new and strong relationship with our EU partners based on the principles of co-operation, collaboration and mutual benefit.
"A future which preserves our ability to trade in goods and services with our biggest market.
"A future that values joint scientific, educational and cultural work with our EU partners.
"A future which allows the UK to retain its leading position in the world, influencing and contributing to developments across Europe and beyond."
Labour's poor performance in the Richmond Park and Sleaford and North Hykeham by-elections has been blamed on the focus on Brexit, and Sir Keir will argue that the Opposition needs to fight on that key political battleground.
He will say: "The battle between these two versions of our future is the battle of our times. It will be fought out over the next few years. The Opposition needs to be in that battle. Labour needs to be in that battle.
"If we are not, the chance to shape the future of our country will be lost.
"Future generations will not forgive us for such a dereliction of duty."
A Conservative spokesman said: "Labour suggested they would support the Government unconditionally in triggering Brexit talks. But behind closed doors they talk about second referendums and now seek to attach conditions and tie the Government's hands.
"The truth is Labour just don't believe Britain can thrive outside the EU, and keep looking for any new excuse to try to block Brexit and overturn the decision of the British people. They're out of touch with the concerns of ordinary, working people."