Lambeth's overview & scrutiny committee will this week probe the borough's controversial decision to allow the Garden Bridge project to use publicly-owned land on the South Bank.
The council is the freeholder of the plot on the South Bank where the Garden Bridge would have its southern landing. The land is currently leased to Coin Street Community Builders, and the report approved by Cllr Hopkins paves the way for the lease to be varied to allow a sub-lease to the Garden Bridge Trust.
Now, as Thursday night's committee hearing approaches, full details of the grounds for the challenge to the decision have been published.
In a strongly-worded critique of their party colleagues, they have accused the council leadership of structuring the decision-making process for the Garden Bridge so that deals can be signed off behind closed doors without public scrutiny.
In their call-in request, the councillors wrote: "The decision as presented is unsound and lacks the requisite transparency.
"The Garden Bridge project is under intense external scrutiny and we consider that it is not only essential to take the decision in accordance with sound procedures with appropriate oversight but also in order to protect the interests of Lambeth Council, its officers and members, the same must be abundantly clear to objective observers.
"Of fundamental importance is that the decision as presented is based on an early draft of the heads of terms, which have been rejected by key parties.
"Moreover, the draft of the heads of terms and the supporting report has a significant amount of errors, misrepresentations and uncertainties upon which such terms depend to be effective.
"It is highly relevant to note that this key decision is being presented for approval by one cabinet member only. It is furthermore unclear and unexplained why the decision has been delegated to the member with responsibility for jobs and growth, whereas this matter falls under the remit of the deputy leader with responsibility for finance.
"We are also very concerned that it defers the next stages of negotiations, which are significant, to Lambeth officers and thereby is structured to avoid further scrutiny."
The full call-in form gives further detailed reasons for demanding a rethink of the decision.
The call-in will be considered by the overview & scrutiny committee on Thursday 21 April at 7pm.
The meeting, which is open to the public, will take place at the Springfield Community and Health Centre in Union Road SW8.
The committee includes eight Labour councillors and one Conservative.
• We hope to provide a live audio or video stream of Thursday's meeting – keep an eye on our @se1 Twitter account for details.