Image caption Lord Falconer said EU membership was a red line for his staying in the shadow cabinet

A member of Jeremy Corbyn's new shadow cabinet has threatened to quit if the Labour leader campaigns for the UK to leave the European Union.

Lord Falconer said he believed the UK should remain a member "come what may" and his position would be "impossible" if Labour's leadership said otherwise.

Mr Corbyn has said David Cameron should not have a "blank cheque" in his negotiations ahead of a referendum.

Unions warn they may argue for an "Out" vote if workers rights are eroded.

Mr Corbyn, who has taken a Eurosceptic stance in the past, has faced questions from his own MPs over his stance on the EU ahead of referendum due by 2017.

He has signalled he will support a vote to stay in but warned David Cameron must not be given a "free hand" in talks and indicated Labour should only support the "right changes" in areas such as social and employment rights.

New shadow chancellor John McDonnell has reinforced this message, by saying it is "fairly reasonable" for the opposition to see what package the prime minister secures before determining its position.

"At the moment it's trying to get a good Europe, trying to get a Europe that serves all our interests, and I think we can do that," he said on Monday.

But Lord Falconer said he could not support any equivocation about the UK's future in Europe.

Media playback is unsupported on your device Media caption John McDonnell: "Reasonable" for Jeremy Corbyn to see what package the PM gets

"If the Labour Party adopts a position which says we might leave the EU and might argue against it then of course my position would become impossible at that point," he told BBC Radio 4's World at One. "But that's not the current position."

The Labour peer said he could envisage "no circumstances" in which it would be beneficial for the UK to leave the EU.

"My view is we should stay in the European Union come what may. So whatever the result of the negotiations, I believe that Britain would be so damaged by leaving the European Union - or indeed saying it's going to leave the European Union - that we should stay."

'Bulwark of support'

Mr Corbyn did not mention the issue of Europe during his speech to the TUC Congress on Tuesday.

The congress is expected to pass a TUC General Council Statement later that says: "We are issuing a warning to the prime minister: you will lose our members' votes to stay in the EU by worsening workers' rights."

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Over the past 20 years, Labour MPs openly calling for Britain to leave the EU have been on the fringes of the party but one of their number, Kate Hoey, said the situation was changing and there were members of the shadow cabinet who would quit if "fundamental reform" was not achieved.

"The dam has opened and we can now have proper debate with people being listened to on all side," she told BBC News.

The leader of the UK's largest union Unite, Len McCluskey, said the union movement had been a "bulwark" of support for the European Union but he was "nervous" about the prime minister's negotiating strategy and what it meant for rights to public holidays, rest breaks and limits on working hours.

"We support, on balance, staying in Europe and that key question of on balance is based on the social charter," he said. "We hope the prime minister takes a step back from that and we will remain a pro-European union."

But the BBC's assistant political editor Norman Smith said there was a serious prospect of the union movement "switching sides" on the issue.

Shadow foreign secretary Hilary Benn has attempted to calm fears among many Labour MPs on the issue, insisting that Labour "has always been committed to not walking away, but staying in to work together for a better Europe".

A number of former shadow ministers have refused to serve under Mr Corbyn since his election, citing his views on the economy, defence and Britain's place in the world.

Chuka Umunna ruled out joining the shadow cabinet after claiming he had not received the assurances he was seeking from Mr Corbyn on Britain's continued membership of the EU.