One hundred prison officers walked out after a guard was sacked for a "pre-emptive strike" on an inmate.
The HMP Liverpool officer, who has 20 years of experience, was forced to act during an "unsafe" incident, the Prison Officers' Association (POA) said.
The union said officers returned to work on Thursday after the prison governor agreed to a review of how she interprets rules on using force.
The Prison Service said the walkout was "unlawful" and threatened legal action.
An urgent interim High Court injunction to prevent the POA from "inducing or encouraging its members to withdraw their services" was sought by the Ministry of Justice before it was withdrawn when the POA instructed its members to return to work.
Staff from other prisons were drafted in to Liverpool after POA members refused to work following a meeting on Thursday morning, the day after the officer was sacked at a disciplinary hearing.
The union had earlier said the HMP Liverpool branch committee "no longer felt safe or confident" in the training members had been given after the governor refused to accept that the use of an "approved method" of restraint was justified.
POA spokesman Glyn Travis said the incident happened last year and disciplinary proceedings ended on Wednesday with the officer's dismissal.
The union said it would continue to appeal against his sacking.
The officer was cleared of any wrongdoing by a police investigation into the incident, said the POA.
Talking about the incident, Mr Travis said: "He felt he and his colleagues were unsafe and responded with a pre-emptive strike.
"Officers believed that this was an acceptable way to deal with the situation but they have not been backed by the governor."
POA assistant general secretary Mick Pimblett said: "We have to take it into context HMP is a violent prison. These staff are facing assaults every single day."
Conditions at HMP Liverpool, which was built in 1855, were described as "squalid" in a 2017 inspection which found rats and cockroaches were rife in parts of the prison and revealed half of inmates felt "victimised" by staff.
A Prison Service spokesman said: "Prison officers have the right to use proportionate pre-emptive force, and related disciplinary investigations only take place where there are concerns that force could have been excessive and potentially unlawful."