The second tube driver at the centre of a strike row was unfairly dismissed by London Underground (LU), an employment tribunal has now ruled.

The Rail, Maritime and Transport (RMT) union had insisted that Arwyn Thomas – who was sacked by LU for “unacceptable and abusive behaviour towards colleagues” – was actually dismissed for his trade union activities, and had called industrial action in his support.

But Transport for London (TfL) – the body responsible for London Underground – said the tribunal found that Thomas “was not dismissed for his union activities, contrary to the claims of the RMT leadership”.

Next week’s walkouts on the capital’s tube network now hang in the balance, as both sides meet for negotiation talks about his reinstatement.

RMT general secretary Bob Crow said: “Throughout this process LU have said that they would abide by the tribunal ruling, so all that we are now asking them to do is stick to their word and get Arwyn back to work. I am seeking an urgent meeting with the managing director to make the necessary arrangements.”

However, Mike Brown, LU’s managing director, said that while the organisation would be “considering the findings in detail”, the tribunal had not called for Thomas’s re-employment.

He explained that the panel had found that Thomas was “50 per cent blameworthy for his dismissal”. Thomas’s explanation for his actions captured on CCTV footage was “not credible”, according to the tribunal.

LU was right to discipline Thomas, said the ruling, but it should have explored options other than dismissal.

The conclusion of this latest case follows the successful unfair dismissal claim of Eamonn Lynch, who LU had sacked for breaching safety rules. He was reinstated last month following the employment tribunal’s judgement, and the RMT consequently cancelled strike action scheduled for May and the beginning of June.

The RMT also suspended industrial action planned for last week while Thomas’s case continued. Should the subsequent wave of walkouts go ahead, they will take place between 9.01pm on 27 June and 11.59am on 28 June; between noon on