The Extinction Rebellion protest could force the Queen to abandon carriage trip to open Parliament, police have suggested.
Officers have told those leading the demonstration that state opening cannot take place if they are camped on the streets as Scotland Yard admitted that they have “contingency plans”.
As demonstrations shutting down the roads around Westminster continued into a third day an additional 500 officers were brought in from forces across England and Wales.
The Metropolitan Police said that it was victims of other crimes who were paying the price of their attention being diverted by climate change protests.
More than 600 Extinction Rebellion protestors have been arrested and eighty tonnes worth of equipment has been seized.
It came as Stanley Johnson, the Prime Minister’s father, addressed demonstrators to offer his support, telling them they “have exactly the right thing in mind here”.
When asked about Boris Johnson’s description of Extinction Rebellion as “crusties”, Mr Johnson said: “I wear that badge with pride.”