Labour's Shadow Brexit Secretary Sir Keir Starmer will unveil six tests for the UK government in a speech at the Chatham House think tank in London on Monday (27 March).
Starmer, speaking just two days before Prime Minister Theresa May plans to invoke Article 50 of the Lisbon Treaty and trigger Brexit talks, will argue that the national interest lies in a "partnership alongside" the EU.
"A partnership based on shared values, common aims and mutual benefit," he will say.
Starmer will argue that such a relationship must start with a comprehensive free trade agreement between the EU and UK, while retaining cooperation in areas such as security, science, research, culture and technology.
"There is a worrying and increasingly powerful move on the Government benches to sever our links with Europe. This is the authentic voice of the Brexiteers," the Labour spokesman will say.
"Those who have argued for decades that exiting the EU offers a once in a generation chance for Britain to extricate herself from the entire European social and economic model: Employment rights. Environmental protections. Fair corporate tax rates. Investment in public services such as the NHS."
He will conclude by warning that Labour will not support a final Brexit deal in the House of Commons if such an agreement fails to reflect the party's six tests.
The comments come after thousands of people marched on the Houses of Parliament on Saturday in an anti-Brexit demonstration. But despite fears that a so called "hard Brexit" could leave the UK worse off, May has promised to press ahead with her plan not to seek full access to the EU's single-market.
The prime minister is visiting Scotland on Monday as part of a UK-wide tour before she invokes Article 50. "We stand on the threshold of a significant moment for Britain as we begin the negotiations that will lead us towards a new partnership with Europe," May will say.
"And I want to make it absolutely clear as we move through this process that this is not – in any sense – the moment that Britain steps back from the world. Indeed, we are going to take this opportunity to forge a more Global Britain.
"The closest friend and ally with Europe, but also a country that looks beyond Europe to build relationships with old friends and new allies alike."
Starmer's six tests
1. Does it ensure a strong and collaborative future relationship with the EU?
2. Does it deliver the "exact same benefits" as we currently have as members of the Single Market and Customs Union?
3. Does it ensure the fair management of migration in the interests of the economy and communities?
4. Does it defend rights and protections and prevent a race to the bottom?
5. Does it protect national security and our capacity to tackle cross-border crime?
6. Does it deliver for all regions and nations of the UK?