Liberal Leader Matthew Guy says he has written to the IBAC asking the commission to make inquiries after Premier Daniel Andrews was forced to deny this week that he had a secret deal with United Firefighters Union Secretary Peter Marshall.

The government is embroiled in another row involving Mr Marshall, after the union leader reacted with fury to the appointment British fire chief Dan Stephens as the new boss of the Metropolitan Fire Brigade, arguing that firefighters have been betrayed.

In an incendiary radio interview on Wednesday Mr Marshall said “promises had been made” by the Premier and his Emergency Services Minister James Merlino and hinted at revelations to come.

Mr Guy said on Friday that he wanted the IBAC to investigate.

“We are pretty concerned about this,” the Opposition Leader said.

“I’ve never seen anything like it before, where a union leader is literally holding the government to ransom and the government is pretty cagey with its answers.”

Mr Merlino was dismissive of Mr Guy’s criticisms, saying the opposition could not be trusted on the issue of the state’s fire services.

“This is an opposition that lies about faith during a parliamentary debate, this is an opposition that, when they were in government, cut $66 million out of our fire services," the minister said.

“There is nothing that the opposition can say on this matter that can be trusted.”

The Minister insisted that the government had done everything right in the process of hiring a new Chief Office for the MFB.

“It was an entirely appropriate process, it was an exhaustive search, an international search for the best person for that job,” Mr Merlino said.

“Of course there needs to be engagement with all stakeholders, including the union, and that is what is occurring.”