Britain would be forced to accept the European Union's refugee quota if Labour rejoins the bloc.
Before Brexit, the UK was allowed to opt out of the EU's asylum policy.
But now, sources have indicated rejoining the union would require the acceptance of all EU "acquis communautaire" - the complete body of common rights, obligations and legal framework by which which member states abide.
Post-Brexit rules enforce quotas of relocated refugees, and governments which refuse to accept them are penalised by as much as £17,500 per person.
Fresh fears were raised over rejoining the EU when Labour leadership hopeful Wes Streeting claimed leaving the bloc was a "catastrophic mistake".
He has said he would seek a "new mandate" for reversing the will of the people in 2016 as he laid out his desire to stand in a looming Labour leadership battle.
Asked if Britain would have to accept the migration pact, an EU diplomat told The Telegraph: "They could opt for the possibility to buy off the relocation, as per the rules of the migration pact which comes into force by June.
"Because, indeed, they would have to adhere to all of the EU acquis, including the stuff we adopted while they were out."
Britain would be forced to accept asylum seekers if it were to rejoin the EU
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Sir Ivan Rogers, the UK's ambassador to the EU from 2013-17, said there would be an "expectation" the UK joins Europe's asylum and migration pact.
He asked: "So the question becomes what does that look like? What does solidarity mean? What's negotiable in numbers? Can you wrap that up in a more expansive pact, which demonstrably puts much more European effort into tackling small boats than we currently have?"
Sir Ivan added: "Are there any viable negotiable halfway house options of serious tangible economic and security benefit between where we are now and membership? The honest answer is basically no."
Andy Burnham, speaking at the Labour conference last year, had admitted he hoped to see Britain rejoin the EU.
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Wes Streeting said he hoped to receive a 'new mandate' to rejoin the EU as he declared he would stand in a future Labour leadership contest
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The Greater Manchester Mayor is understood to stand by those remarks, but has said he was "not advocating that in this by-election", adding that there was a "long term" case to rejoin the bloc.
In 2016, Makerfield voted overwhelmingly to leave the EU.
Last year, Sir Keir agreed to a "one in, one out" deal with Emmanuel Macron, but hopes for an EU-wide deal have not come to fruition.
That deal has forced British taxpayers to shell out £50,000 to deport a single "asylum seeker"
Only 604 have been returned to France.
The EU has refused a bloc-wide 'one-in one-out' deal, the likes of which Britain has signed with France
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The UK left the Dublin Regulation, which allowed EU members to return asylum seekers to the first safe EU country they arrived in, after Brexit.
The Dublin Regulation is set to be replaced by the EU's new migration pact, officially named the Asylum and Migration Management Regulation - which will come into effect next month.
Labour had previously considered joining the EU migrant quota scheme in exchange for a broader migrant return deal, but the offer was never formally made.
EU officials calculate the number of migrants with a "solidarity mechanism" which measures both population size and total GDP.
There is a mandatory "solidarity pool" of at least 30,000 relocations across the EU, with Germany pledging to accepting 4,555 relocated asylum seekers, and France promising to accept 3,000.
Denmark has opted out of the quota system, while Ireland has chosen to opt in.
Both Hungary and Poland have refused to accept refugees, and face fury, fines and legal action from Brussels as a result.