Hackney council bosses have reneged on an agreement with drivers and passenger escorts, members of the Unite union, on school buses for disabled children.
Unite said that Hackney's council's u-turn for its 38 members now meant that future industrial action was very much back on the table.
The workers called off their strike on 9 October in a long-running dispute over payments for working split shifts. But now the council is insisting that the staff use contractual leave days for the training days.
Unite regional officer Onay Kasab said: "We reached a settlement that agreed the training and development sessions would take place during paid work time. This was a vital component that paved the way for our members to call off their strike action.
"However, when we met council officers to discuss implementation of the agreement, we were told that staff would need to use their 'inset days' for training as part of the arrangements. For clarity, the inset days for these staff are not used for training - they are paid leave days.
"Clearly, the council is unhappy that staff receive paid inset days which are, in effect, contractual leave days. But this is a completely different issue and not linked to this dispute.
"The employer's actions in this case do not just demonstrate bad faith as far as negotiated agreements are concerned - they demonstrate appalling opportunism.
"Unless the employer abides by what was clearly understood, negotiated and agreed then we will restart the industrial action campaign, including strike action."