Inspectors Matthew Birkinshaw and David Troy set out their inital findings on the Chiltern and South Bucks Local Plan in a letter this week, following the start of the document's examination.
The duty to cooperate requires local planning authorities preparing development plans to engage constructively and on an ongoing basis in the relation to cross-boundary strategic matters, such as meeting housing need.
While the duty is "not a duty to agree", the letter states, the Planning and Compulsory Purchase Act 2004, which enshrines the mechanism, is "clear that strategic plan making authorities should engage constructively, actively and on an ongoing basis".
The inspectors said the key strategic issue in this case was accommodation of Slough’s unmet housing needs.
The 2017 Slough Local Plan issues and options document described the shortfall at "around 8,000 dwellings", according to their letter.
It says that it was "clear from evidence before the examination that Slough Borough Council’s ability to meet its own housing needs has been a longstanding matter for discussion" and was "before the councils at a very early stage in the plan’s preparation".
But the inspectors said that, from dialogue in October 2017 and January 2018, "no further meetings were held with [Slough Council] for almost 18 months, until after publication of the submission version local plan".
"Despite [Slough Council] raising concerns once more, no formal notes of the meeting were taken, and the plan proceeded to submission in September 2019.
"In our opinion, this does not demonstrate that the councils have taken reasonable steps to engage actively, constructively and on an ongoing basis in relation to the strategic issue of unmet housing need."
Chiltern and South Bucks councils also failed to have a signed statement of common ground in place with Slough, as required by paragraph 27 of the National Planning Policy Framework, which "serves to demonstrate a lack of effective dialogue".
The councils "have only very recently (March 2020) submitted a draft, unsigned statement of common ground", the letter adds.
The inspectors conclude: "Based on the evidence provided we therefore have very serious concerns that the councils have not engaged actively, constructively and on an ongoing basis in relation to a strategic matter in the plan’s preparation as required by Section 33A of the Act.
"Whilst we have sought to be pragmatic in our approach, Section 20(7A) of the Act requires that the examiners must recommend non-adoption of the plan if they consider that a council has not complied with the DTC.
"We will not reach a final conclusion on this matter until the council has had the opportunity to consider our findings and respond to this letter.
"However, it is important that we point out that there is a strong likelihood that the only option will be for the council to withdraw the plan."
As of 1 April, both Chiltern District Council and South Bucks District Council now form part of the new county-wide unitary authority Buckinghamshire Council.
Consultation on the Chiltern and South Bucks local plan took place last summer and the plan was submitted for examination on 26 September 2019.
The plan proposes the release of green belt land to accommodate 5,200 homes, more than a third of the plan’s total housing target over 20 years.
Hearings for stage one of the examination were due to take place between mid- and late-March but were postponed following the coronavirus outbreak.
In a statement on its website, the council said it "will now consider [its] position before responding in due course".
Warren Whyte , Buckinghamshire Council’s cabinet member for planning and enforcement said: “The council has already been facing the prospect of building in some green belt areas to ensure we meet the housing needs of Buckinghamshire residents.
"To suggest that the plan should also have accommodated up to an additional 10,000 houses for Slough is ridiculous.
"I hope when we present out arguments to the inspectors that they are persuaded to change their minds.”
So far this year, three councils have seen their draft local plans derailed at examination because they failed to meet the legal duty to cooperate. If the South Bucks and Chiltern plan suffers the same fate, it would be the fourth.
An article examining why some councils are still struggling to meet the duty to cooperate, including figures on how many plans have failed the duty, can be found here.
In December, a local Conservative MP Joy Morrissey wrote to housing secretary Robert Jenrick requesting that the Chiltern and South Bucks local plan be called in, claiming that the strategy was needlessly proposing the development of green belt land.