Boris Johnson has warned the UK is "on the road to serfdom" amid reports Sir Keir Starmer will start to renegotiate the post-Brexit deal with the EU within weeks.
The new Prime Minister is expected to use a major summit at Blenheim Palace later this month to unofficially get the ball rolling.
Sir Keir is said to want lower trade barriers and have more relaxed freedom of movement rules. EU sources believe he is willing to sign Britain up to some of Brussels's rules in exchange.
But Mr Johnson warned against any renegotiation of the five-year pact he signed in 2020.
The former prime minister told the Mail on Sunday: "The great sell-out begins.
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"Behind these harmless-sounding agreements is the reality that the UK will be accepting rules set by Brussels – no matter how onerous – with no UK say on the making of those rules.
"We are on the road to serfdom under Starmer."
The European Political Community summit in Oxfordshire on July 18, which was put in the diary under Rishi Sunak's premiership, will bring together about 50 leaders from across Europe.
A Brussels source said: "We expect this to be the moment at which Starmer unofficially sets out his offer on trade to the big EU players."
Sir Keir, who backed Remain and a second referendum, has previously said he would seek a better deal with the EU if he won power.
But the Labour leader has insisted he would not drag the UK back into the Single Market or customs union.
A Labour spokesman said trade negotiations are "not on the agenda" of the summit.
He added: "It is not the right forum for those conversations. There will be no return to freedom of movement."