The mayor of London will draw up a new charter regulating the management of privately owned public spaces, following a Guardian Cities investigation which uncovered growing corporate control over parks and squares in the capital.

The announcement comes as Sadiq Khan prepares to publish the first draft of his London Plan – the document that sets out the mayor’s strategic vision for London, and shapes development and planning policies across all of the city’s local authorities.

The charter, the first of its kind in London, will set out both rights and responsibilities for users and owners of public spaces, regardless of whether they are council-run or in the hands of private developers. Earlier this year, the Guardian revealed an almost complete lack of transparency over the rules governing “pseudo-public spaces”: open land that is accessible to citizens but is actually classed as private property, enabling security guards to eject members of the public on a whim and bar them from taking photographs or holding protests.

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Pseudo-public spaces can now be found at some of London’s most famous locations, from the vast new redevelopment surrounding King’s Cross station to the site of City Hall itself, home to the capital’s democratic institutions and yet ultimately owned by the sovereign wealth fund of Kuwait. The revelations sparked a political backlash, with party leaders Jeremy Corbyn, Vince Cable and Caroline Lucas all speaking out to demand better protection of the public realm. Other leading mayors, including Andy Burnham in Manchester and Steve Rotheram in Liverpool, also stepped in to promise renewed protection of public spaces in their cities.

“The new London Plan will propose that effective management and ongoing maintenance of [the] public realm should be a key consideration in the design of places and secured through the planning system where appropriate,” Khan told the Greater London Authority recently, after it passed a motion calling on the mayor to address concerns, and announced plans to hold an inquiry into the issue. “Whether publicly or privately owned, [the] public realm should be open, free to use and offer the highest level of public access.”

Siân Berry, the Green Party assembly member who first proposed the motion, offered a cautious welcome to Khan’s plans. “The corporate takeover of our public spaces and their culture of secrecy might finally be stopped short,” she said. “But what this new public London charter means remains unclear. The charter sounds like it will only be advisory, so the real test will be in new policies and planning that must keep our public spaces open and inclusive for all.”

Facebook Twitter Pinterest Regent’s Place in London, owned by the commercial property company British Land. Photograph: Tom Jenkins/The Guardian

Berry pointed out that in Camden, where she is a local councillor, new draft planning guidance will commit the council to creating open spaces that are “welcoming for everyone” and ensure that any rules drawn up by the owners of pseudo-public spaces are transparent, accountable and justified by the same reasons as those covering local authority bylaws. It will also introduce “break clauses” in the council’s contract with property developers, allowing the council to take over pseudo-public spaces if there are concerns with their management – with the cost of maintenance still borne by the developer for 10 years.

“I think this is an example of good practice which the mayor of London should take notice of,” argued Berry. A spokesperson for Sadiq Khan told the Guardian: “The mayor believes London’s public spaces should be open and free to use.”