Jeremy Corbyn has said the UK needs to "look very carefully at the terms" of any future trading relationship with the European Union.
The Labour leader is facing renewed calls to commit his party to keeping the UK in the EU single market and customs union after Brexit, ahead of its annual conference in Brighton.
Speaking to the BBC's Andrew Marr Show, Mr Corbyn said he wanted to ensure "tariff-free access to the European market".
But he also stressed the need to "look very carefully at the terms".
"At the moment we are a part of the single market and that has within it restrictions on state aid and state spending and pressures on it, through the European Union, to privatise rail and other services.
"I think we need to be careful about the powers we need as a national government."
Mr Corbyn also suggested that EU rules could block a future Labour government from investing in industries.
Thirty senior figures, including a close Corbyn ally, have written an open letter to the party leader saying the party needs to go further to protect jobs and workers' rights after the UK leaves the EU.
The signatories to the letter published in The Observer include former shadow cabinet members Chuka Umunna and Heidi Alexander, as well as one of Mr Corbyn's closest allies in his early days as leader, Clive Lewis,
The letter, which was also signed by the TSSA union's general secretary, Manuel Cortes, former Northern Ireland secretary Lord Hain and Liverpool mayor Joe Anderson, said Labour needed to present an alternative to the Tories' "destructive Brexit".
Shadow Brexit secretary Sir Keir Starmer has previously said that under a Labour government Britain would remain in the single market and the customs union for a transitional period of two to four years after Brexit.
"The supposed benefits of a clean break with the EU are a fantasy. The economic impact of leaving the single market would hit the most vulnerable in our society hardest," the letter said.
"So at our conference this week, Labour should commit to staying in the single market and customs union - ruling out no options for how to achieve this - and to working with sister parties and others across Europe to improve workers' rights, boost trade union membership and put an end to the exploitation of workers, not freedom of movement.
"This would send a powerful message of solidarity to the rest of Europe, and to the millions of EU and UK nationals living in limbo here and across the continent."
When asked about the fact that a large majority of Labour party members want free movement to continue, Mr Corbyn told Andrew Marr: "I understand the points they're making, and I understand the importance of workers' movement from one place to the other.
"But I also understand that there is an abuse of free movement by some employers who have grotesquely exploited some very low-paid workers, that has to stop."
He added: "But we have to recognise that in the future we're gonna need people to work in Europe, and people from Europe are going to need to work here. There's going to be a lot of movement."
On Friday, the prime minister delivered a landmark speech setting out her plans for Brexit.
Theresa May said she wants the UK to have a two-year transitional period after Britain leaves the bloc in 2019.
Under Mrs May's proposals, the UK could pay around £18 billion into the EU budget during the transition period - enabling access to the single market.
EU nationals would also remain free to move to Britain, although they would have to register with the UK authorities.
Mr Corbyn said a transitional period should last "as long as necessary" as he questioned Mrs May's suggestion it should be around two years.
Asked whether it could last as long as a decade he said "no, I don't think so" but added: "I think it's impossible for anyone to put an absolute figure on that."