Britain will have to strike a balance between preserving access to the single market and the free movement of people, if the EU insists on it, according to Jeremy Corbyn.
The Labour leader, who is expected to say the UK could be "better off" outside the EU in a speech later today, said that economically Britain will have to be able to trade with Europe.
Corbyn also criticised employers in the UK who were "under-cutting" working conditions by employing cheap migrant labour.
The Islington North MP told Good Morning Britain if employers played fair then the numbers coming to Britain to Europe would likely decrease.
But asked if he would accept continued free movement as the price of access to the single market, the Labour leader said: "If the EU, as is, says access to the single market requires the continuation of free movement, then there's a choice to be made."
He added: "I would say we have to end undercutting and exploitation and that would in turn, probably, affect numbers."
Economically "we have to be able to trade with Europe", Corbyn said.
But he added that migrant workers were vital to Britain, saying the UK "wouldn't have much" of an NHS without the force.
"There is a big economic decision that's got to be made then," he said.
"If we want to retain jobs in this country, if we want to retain the trading nature of our economy and the ability to export to Europe, then a deal has to be done with the EU in order to achieve that."
Corbyn also insisted to GMB he wants to "be in government" and sort out Britain's housing crisis.
The Labour leader rejected the idea he preferred to remain in opposition as a "serial protester-objector," and maintained he wanted to be in power.
"I want to be in government, so we can conquer the housing crisis in Britain", he said.
"I want to be in government so that young people get a real chance in life.
"I want to be in government so we don't become a country that's one of the most unequal in Britain, if not in the OECD countries, one that actually gives a real chance to everybody."
Asked whether he believed Labour could win a general election under his leadership, the 67-year-old responded: "Yes, absolutely."