Video report by ITV News Political Editor Robert Peston
Labour have pledged to ditch the government's "rigid" and reckless" Brexit strategy and guarantee the rights of European Union citizens "on day one" if elected.
Shadow Brexit secretary Sir Keir Starmer outlined Labour's vision for Britain's withdrawal from the EU in a speech, saying: "Brexit doesn't have to mean whatever Theresa May says it means."
Retaining the benefits of the single market and customs union would also become top priority in Brexit talks if the party is victorious on June 8.
The prime minister later dismissed Labour's Brexit proposals as "nonsensical", adding: "What we've seen today from Labour is, I think, their seventh Brexit plan."
Sir Keir said Labour accepted immigration rules would have to change when Britain leave the EU, but would move immediately to protect the right of EU workers in the UK and British workers in the EU.
ITV News Political Correspondent Emily Morgan noted the pledge was warmly received.
"EU nationals do not just contribute to our society: they are our society. And they should not be used as bargaining chips," he said.
"On day one of a Labour government we will immediately guarantee that all EU nationals currently living in the UK will see no change in their legal status as a result of Brexit, and we will seek reciprocal rights for UK citizens in the EU."
Sir Keir said Labour would enhance the rights of citizens and workers by replacing the Great Repeal Bill with an EU Rights and Protections Bill.
Sir Keir Starmer also said the option of remaining in the EU customs union - which would severely restrict the prospect of independent UK trade deals - should be part of withdrawal negotiations.
"What we should do is make sure we have got options on the table, not take options off the table," he said, citing the customs union as a "classic example".
He went on: "There is anxiety across the manufacturing sector about the impact of simply walking away from the customs union.
"If we leave it on the table, it may be in a year, or two years, that doesn't work - but why take options off the table before you have even started negotiations?""
The prime minister has said the UK "cannot possibly" remain part of the single market as it would mean "not leaving the EU at all".
Answering questions during a campaign visit to Bridgend in South Wales, the Conservative leader said Labour's proposals were "yet another nonsensical Jeremy Corbyn plan for the future in terms of Brexit".
She added: "What you need is a government which has a clear plan for those negotiations. That's what the Conservative Government has. It's what the Conservative Party has."
Leaders across the bloc have warned Britain cannot enjoy the benefits of remaining in the single market without agreeing to the rules, such as freedom of movement.
But Sir Keir said: "The white paper will have a strong emphasis on retaining the benefits of the single market and the customs union as Labour know that is vital to protecting jobs and the economy.
"And we will approach negotiations in a completely different way to a Tory Brexit: negotiating for the many, not the few.
"Where Theresa May wants to shut down scrutiny and challenge, Labour will welcome it.
"We will work with Parliament, not against it. Because on an issue of this importance, the Government can't hide from the public or Parliament.
"A Labour approach to Brexit means legislating to guarantee that Parliament has a truly meaningful vote on the final Brexit deal."