On 28 February, Sustainable Development Capital (SDCL) announced it had launched a judicial review of the government's decision to award preferred bidder status to another party.
A preferred bidder has yet to be announced, but is widely understood to be Australian banking group Macquarie.
SDCL claimed the preferred bidder's bid was not compliant with the government's own criteria and said the fact that no deal was completed within the target timetable "attests to the fact that the preferred bid was neither deliverable within the timeframe nor acceptable."
It said it has longstanding concerns with the way the Department for Business, Energy and Industrial Strategy (BEIS) has been handling the sale process.
These had been raised "since the final phase of the auction in September 2016", SDCL said, but "in the absence of a constructive dialogue, we have no alternative but to seek redress through the judicial review process".
SDCL argued its own bid matched the key criteria for the sale, stating it was designed from the start to provide "value for money for the UK taxpayer, transaction certainty and continued investment in the green economy".
The judicial review is only the latest of several setbacks for the divisive sale of the bank, with the political heat on both BEIS and Macquarrie Group growing since the process formally began.
Opposition MPs, including House of Commons Environmental Audit Committee (EAC) chair Mary Creagh, Green Party MP Caroline Lucas, former business secretary Vince Cable and former Labour leader Ed Miliband, have warned that the bank could be asset-stripped and broken up by the preferred bidder, threatening its green purpose.
The GIB's broad green objectives after sell-off are protected by a group of independent trustees appointed in November 2016 and a special share controlled by the Green Purposes Company.
But these powers do not extend to day-to-day decisions and it is unclear how subsequent restructurings and sell-offs would be protected.
In January, pressure in the House of Commons increased for a review of the sell-off and the protection of GIB's core purpose.
There were also rumours that a public share offering could take place instead. SDCL has said it would support such a move within three years, after the bank's investments achieve economies of scale and value for money for taxpayers.
An integral GIB is seen by objectors as increasingly important to the government's industrial strategy, particularly after loss of other infrastructure funding post-Brexit. Both BEIS and Macquarie declined to comment on the matter.
The application for judicial review will be heard in the High Court in March.
First published on ENDS Report