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Protestors camped outside St Paul's cathedral have been told to leave by 6 tomorrow night or face legal action.
Notices were tied to the tents of campaigners ordering them to pack up by 6pm on Thursday.
The corporation said it would issue proceedings in the High Court if they didn't meet the deadline.
But the Occupy London protesters immediately vowed to fight the decision.
The corporation says it decided to resume legal action after two weeks of talks with protesters "got nowhere".
It had previously offered to let the camp remain in place until the New Year.
Stuart Fraser, policy chairman of the City of London Corporation, said on Tuesday: "We paused legal action for two weeks for talks with those in the camp on how to shrink the extent of the tents and to set a departure date - but got nowhere.
"So, sadly, now they've rejected a reasonable offer to let them stay until the new year, it's got to be the courts.
"We'd still like to sort this without court action but from now on we will have to have any talks in parallel with court action, not instead."
Naomi Colvin, an Occupy London spokeswoman, said: "It is not something we need to be remotely worried about - we've been prepared for it for months.
"If they want to get an order in the High Court it could take months. We will contest it.
"We will be speaking to our legal team and we will be fighting it."
She questioned the corporation's motives for asking the protesters to leave, adding: "The area by the side of St Paul's Churchyard is not obviously in anyone's way - people move freely up and down it every day.
"They say it is a highway but there haven't been horses and carts here for hundreds of years.
"People are so serious about this crisis that they are willing to leave their homes and camp out during the English winter."