Andy Burnham's plans to reverse Brexit could push up immigration by more than 200,000 a year it was claimed - after Emmanuel Macron told him Britain would have to accept free movement.
Whitehall sources told the Mail that the French President has warned Mr Burnham he will have to accept open borders with the EU if he wants to rejoin the single market or become a full member of the Brussels club again.
New analysis by Reform UK suggests the move would open the door to an extra 213,000 migrants a year, based on pre-Brexit figures.
The Prime Minister was accused of betrayal this week after setting out plans to unpick the Brexit settlement.
In a speech to Labour activists at the party's annual conference in Liverpool, he said he would use a UK-EU summit this autumn to open a debate about Britain's relationship with the EU.
He said potential options could include joining the single market or customs union or going 'all the way' and rejoining the EU, with or without a second referendum.
Mr Burnham claimed that Brexit had 'weakened' immigration control - but made no mention of the likely return of free movement, which gave EU citizens the untrammelled right to live and work in the UK.
Reform UK said reviving free movement would trigger an immigration surge.
French President has warned Mr Burnham he will have to accept open borders with the EU if he wants to rejoin the single market or become a full member of the Brussels club again
The party said it was likely to boost arrivals by more than 200,000 a year - and could be far higher.
Nigel Farage said: 'This is Brexit betrayal writ large. Burnham wants to open the borders to nearly half a billion people with no way of controlling who enters the country.
'It's clear that Open Borders Burnham doesn't care about the British public's concerns about immigration.'
The 213,000-a-year figure is based on the average number of arrivals from the EU in the period 2012 to 2019, which spans the referendum.
Arrivals were lower at just 75,000 in the final year of free movement. But they averaged more than 270,000 a year in the two years running up to the referendum.
The Centre for Migration said it was 'important that the public are aware of the risks associated with a return of freedom of movement rules'.
In a statement, the think tank added: 'There is no doubt that rejoining the EU would lead to a spike of migration from the continent, especially the likes of Romania and Bulgaria, reversing several years of European net negative migration.
'There is a danger that inward migration surges, especially in the absence of labour market reform, and the British government would be simply unable to do anything about it.'
The Mail can reveal that President Macron warned Mr Burnham last month that he would have to accept free movement if he wanted a much closer trading relationship with Brussels.
A Whitehall source said the French President was clear that Britain would have to accept 'all four freedoms' - the ability of goods, services, capital and people to move freely across the EU's borders.
Experts have warned the UK would also have to sign up to join the euro in the long term, as well as losing the annual budget rebate negotiated by Margaret Thatcher.
Free movement was a major issue during the 2016 referendum. But some ministers believe the post-pandemic wave of non-EU migrants is more controversial - and that reviving free movement would restore the right of British citizens to live and work in the EU.