Transport union leader Mick Cash has been barred from taking part in peace talks on the Southern Railway dispute at Acas.
The Rail, Maritime and Transport (RMT) union leader had been due to join talks with officials from Southern to seek a resolution in a bitter dispute over driver-only trains.
Mr Cash said that the company had "refused" him access to the talks, which also involved the Aslef union. He branded their behaviour as "absolutely disgraceful" and "counter-productive" today.
Southern's parent company earlier today said that they had offered to meet Mr Cash "later today" over the dispute.
A planned strike went ahead despite the talks, with another 24-hour stoppage planned on Friday, and a week-long walkout planned in the new year.
Transport Secretary Chris Grayling has refused to rule out taking action to prevent future stoppages like the one which brought all Southern Railway services to a halt.