It comes after viral footage online showed far-right activist Tommy Robinson walking by a guy lying on the ground at the station shouting, “he come at me”.
Police were called to St Pancras on Monday (July 28) just after 8.40pm to reports of an assault.
A man was taken to hospital with serious injuries, which since are not thought to be life threatening.
Officers arrested a 42-year-old man from Bedfordshire yesterday (August 4) on suspicion of grievous bodily harm.
In a new update from the British Transport Police, he has been released on police bail until September 2 while enquiries continue.
The arrest was at Luton Airport shortly after 6.30pm, after officers were notified that the suspect had boarded an incoming flight from Faro.
BTP said the man had been wanted for questioning aft