A key vote in the European Parliament later could see an end to Britain's opt-out of European laws limiting the working week to 48 hours.
Labour MEPs may vote to end the opt-out against the wishes of Prime Minister Gordon Brown who wants to keep it.
The European Commission has said the opt-out should stay arguing it is now used by at least 14 other countries.
Thousands of trade union members marched on the European Parliament to urge an end to the opt-out.
And leaders of the UK's biggest union, Unite, urged British MEPs to "stop the UK's long hours culture".
But on Monday Business minister Pat McFadden told the BBC it would be a mistake to end the opt-out during an economic downturn when people might need to work extra hours.
The opt-out was negotiated by the Conservative government in 1993 and is used by some other member states.
But in the debate later British Labour MEPs may side with trade unions and back an amendment to review the opt-out in five years' time.
Trade unions and businesses have been lobbying MEPs up to the last minute. Labour MEPs met in Strasbourg on Tuesday night to decide whether to continue their opposition.
BBC Europe editor Mark Mardell said they were split on the issue "and some sound rather tortured about their decision". Formally they have a meeting at 0900 before the vote a couple of hours later, but
"I think many will do what they want to do, whatever the group decides as a whole," said Mr Mardell.
Gary Titley, leader of the Labour group in the European Parliament, said: "We are looking at how the different sides of the argument can be brought together and are trying to find a way to allow for British workers to have maximum flexibility while ensuring the health and safety of workers and consumers alike."
Conservative and Liberal Democrat MEPs say the opt-out should continue but Plaid Cymru MEP Jill Evans said she would vote against it and the Green Party's two British MEPs are also expected to vote against it.
Open Europe, which campaigns for EU reform, estimated ending it in 2011 - as some MEPs want - would cost the UK economy between £47.4bn and £66.45bn by 2020.
CBI deputy director general John Cridland said European Parliament amendments which would stop people being able to choose to work more than 48 hours would "replace opportunity with obstruction".
"If your partner has lost their job, should Brussels stop you from putting in extra overtime to support your family?," he said.
But the unions argue it is a health and safety issue.
Unite joint general secretary Tony Woodley said: "Tired, overstretched workers are not productive workers and are putting themselves and others at risk, such as in the transport industry where we know, for instance, that tired drivers are more dangerous than drunk drivers."
Even if MEPs vote to scrap the British opt-out, it would go on until 2012 and there would be further negotiation between the EU and the British government.
Writing in his blog, Mark Mardell said: "If British ministers and others won't budge that means it's back to the drawing board. As far as I can see that means Britain would keep the opt-out for a good while, although exactly what would emerge in the end is very uncertain."