Anti-capitalist protesters who have been camped outside St Paul's Cathedral for a month must quit their tents or face High Court action.
The City of London Corporation on Wednesday served a legal eviction notice on the activists which gives them until 6pm on Thursday to leave.
However demonstrators, who tore down the notices as soon as they were posted, vowed they would stay and said they were prepared to fight a potentially lengthy legal battle.
The corporation, the local authority which runs the Square Mile, has expressed concern over "worrying trends" at the camp, including late-night drinking, and said companies near the cathedral had complained about losing business.
Some of the tents are pitched on the footpath next to the shops surrounding the cathedral, with the remainder in the churchyard itself.
A spokeswoman for the cathedral - which has ditched its own proposed legal action - said that it was looking for a "peaceful resolution" and said it had faced "challenges" since the camp was pitched a month ago.
The notice said: "If any tents and other structures remain after 6pm on Thursday 17 November 2011, proceedings for possession and injunctions will be issued in the High Court of Justice without further notice. If granted, this would mean that you would be ordered by the High Court to remove the tents and any failure to do so could be a contempt of court."
The corporation decided to resume legal action after two weeks of talks with protesters.
Stuart Fraser, policy chairman of the corporation, said previously: "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."