Jeremy Corbyn will renew his call to the government to guarantee the residency rights of three million EU nationals living in Britain on Saturday afternoon (4 March).
The Labour leader, speaking at an NHS rally organised by the People's Assembly in London, will argue that their talents are needed to help run the health service.
"Let's say this clearly: EU nationals, who have made their lives here, are welcome here. Their rights should be guaranteed. Labour voted for it in the Commons; we beat them in the Lords," he will say.
"And now it is time for Theresa May to guarantee the rights of all EU nationals in this country. People are not bargaining chips.
"Over 50,000 EU nationals, including 10,000 doctors, work in our NHS. Our NHS needs all their talents."
The comments come after the House of Lords voted 358 to 256 to back an amendment to the government's Article 50 bill, the legislation needed to trigger Brexit talks. The draft legislation will now go back to the House of Commons where MPs will vote on the EU nationals amendment.
"We are disappointed the Lords have chosen to amend a Bill that the Commons passed without amendment," a spokesperson for the Department for Exiting the EU said.
"The Bill has a straightforward purpose — to enact the referendum result and allow the government to get on with the negotiations.
"Our position on EU nationals has repeatedly been made clear. We want to guarantee the rights of EU citizens who are already living in Britain, and the rights of British nationals living in other member states, as early as we can."
Corbyn, meanwhile, is also expected to declare that the NHS is in crisis.
"When they've found the money to cut taxes on big businesses, cut taxes on the richest families, cut taxes for the speculators and cut taxes for the fat cats," he will say.
"Don't then tell us there's no more money for our NHS. There is no excuse."
The Conservatives had not responded to a request for comment at the time of publication.