The draft new London Plan was published for public consultation in December 2017 and an examination in public was held between 15 January 2019 and 22 May 2019.
The plan states that development proposed in the green belt should be refused and advises London boroughs that plans to de-designate green belt sites will not be supported while any extensions of such designated land will be backed.
However, inspectors Roisin Barrett, William Fieldhouse and David Smith, in an examination report made public this week, have advised the mayor that a green belt review is required "to at least establish any potential for sustainable development" and called for a commitment that a review will be conducted to inform the next edition of the plan.
They also said the review should "involve joint working with authorities around the administrative boundary as well as the borough".
The report states: "Any exercise should consequently take account of cross-boundary issues relating to the coherence and durability of the green belt on the periphery of the capital as well as across London itself. Therefore, a key part of an effective review in London is likely to involve joint working and positive engagement with adjoining authorities and boroughs."
The inspectors found that the plan’s blanket opposition to de-designation of green belt sites "is not consistent with national policy" which allows green belt development in "very special circumstances" and the alteration of green belt boundaries in "exceptional circumstances".
The inspectors said: "The plan itself observes that some green belt land is derelict and unsightly and does not provide significant benefits.
"In any event it is implausible to insist that the green belt is entirely sacrosanct without having considered what it comprises and the impact that it has on wider strategic objectives."
The inspectors’ findings reflect evidence heard during examination hearings earlier this year, at which groups including the Home Builders Federation and London First argued that the draft London Plan’s green belt position contravened national policy.
Khan must now consider the inspectors’ recommendations before submitting his response and a revised plan to the secretary of state for approval.
London's deputy mayor for planning Jules Pipe said: "The mayor makes no apologies for seeking to protect and enhance the green belt and Metropolitan Open Land – an important natural asset which provides vital environmental and recreational benefits, prevents urban sprawl, and drives the use of previously developed land.
"The plan makes clear that we should build the homes Londoners need, by developing on brownfield land, town centres becoming denser, incorporating more green infrastructure in our streets and developments and using the hundreds of small sites across the capital."
Jonathan Seager, executive director for place at business group London First, said the recommendation of a London-wide green belt review that also involved neighbouring Home Counties authorities was "very significant".
He said: "That's a fundamentally big recommendation, which from our perspective is very sensible.
"It's the first time in recent years that inspectors examining the London Plan have made a detailed recommendation calling for a review of the capital's green belt."
The inspectors also said recommended that the plan cut its proposed small sites housing target by more than half after finding it to be "neither justified nor deliverable".