Jeremy Corbyn will pledge that his party will push for a new relationship with the EU's single market that will include tariff-free access after Brexit.
The Labour leader will also outline in a speech on Monday (26 February) how the UK should follow the example of countries that have close ties to the EU, but are not members, like Switzerland, Turkey and Norway.
He will insist that Brexit "does not inevitably spell doom for our country" and that it is "what we make of it together".
"Britain will need a bespoke relationship of its own. Labour would negotiate a new and strong relationship with the single market that includes full tariff-free access and a floor under existing rights, standards and protections," he will say.
He will add that central to any deal, would be negotiating protections, clarifications or exemptions, if needed, on areas of EU legislation. These include privatisation and public service competition directives, state aid and procurement rules and the posted workers directive.
"We cannot be held back, inside or outside the EU, from taking the steps we need to support cutting edge industries and local business, stop the tide of privatisation and outsourcing or prevent employers being able to import cheap agency labour from abroad to undercut existing pay and conditions," he will tell the audience in Coventry.
While stopping short of directly saying he wants to stay in the customs union, his shadow Brexit secretary Sir Keir Starmer told the BBC on Sunday that Labour would negotiate a treaty that would "do the work of the customs union".
This message has been leapt upon by David Davis, who said that the Labour leader was "selling snake oil" over Brexit.
The Brexit secretary wrote in the Telegraph how Labour's policy would be guilty of two "serious breaches" of its manifesto and jeopardise jobs by surrendering one of the "central prizes" of Brexit.
"Labour may think they have stumbled across a simple solution to Brexit, but there is a lesson they are yet to learn: if it looks like snake oil, and it smells like snake oil, don't expect it to make you feel better," Davis wrote.
But pressure is building up within the Tories after Theresa May ruled out any kind of customs union for fear of a backlash from Eurosceptics in her party who are warning that any row back from this position could put her premiership at risk.
One source told the Telegraph: "They can't keep us in the Customs Union by another name. It would be a hokey-cokey amendment, one foot in and one foot out. It would be a complete sellout. If she does this the letters will go in (for a leadership contest])".