The High Court could make a walkout by the PCS Union unlawful - but I'm told that some border staff plan to start the strike tomorrow.
Those whose shift starts on the 25th but end on the 26th, could walkout before midnight. So the court would have to act quickly, allowing time for its decision to be communicated to the grass-roots
Tonight the Home Office is not giving any details - they refer to this only as "a procedural error", indicating it is not to be alleged that there was deliberate wrong doing by way of vote rigging or similar.
Senior sources I've spoken to at the PCS Union are unclear what the precise Home Office case is.
They do say they will robustly defend their ballot, though there was only a 20% turnout for that strike vote.