Prime Minister Joseph Muscat said on Monday that he will “limit” himself to the day-to-day operational side of running the administration, one day after announcing his resignation, effective January.

In a short statement given to media waiting outside the Auberge de Castille, Dr Muscat said a new prime minister would from January be able to take the major decisions that needed to be taken.

He explained the timeframe between him announcing his resignation and a new PL leader being selected was the shortest possible.

The outgoing prime minister hailed the major breakthrough in the Daphne Caruana Galizia assassination investigation.

On Saturday, Tumas Group magnate Yorgen Fenech was charged with complicity in the bomb plot.

Dr Muscat gave Melvin Theuma, the “middleman” in the bomb plot, immunity from prosecution to testify against Mr Fenech and others believed to be involved in the assassination.

A pardon request by Mr Fenech was turned down. The businessman has claimed the prime minister’s former chief of staff Keith Schembri was a co-conspirator in the murder.

The prime minister said it was obvious there was a perception of shortcomings in the process or that thing could have been done differently by some people.

“I need to shoulder responsibility for that.”

Dr Muscat refused to answer questions, saying his replies could be interpreted as trying to defend someone or undermine the case, 'which I will not do'.

His failure to immediately resign has enraged critics.

Opposition leader Adrian Delia said the country could not afford to wait another day for Dr Muscat's resignation.

"He should have resigned long ago and every day he remains in the post will be another day where justice is not done and far less seen to be done. Joseph Muscat has no legitimacy," Dr Delia wrote on Twitter.

His predecessor Simon Busuttil described the Prime Minister's decision as "unacceptable" and insisted he should resign at once.

"He must not be allowed to continue using his position to obstruct justice and avoid being investigated," Dr Busuttil said. "He has played this game for far too long already."

In a statement, the Chamber of Advocates said the Prime Minister's position was untenable and that he should resign immediately to preserve the credibility and integrity of the investigative process.

It said the conflicts of interest in the case were self-evident and posed a "clear and present danger" to the investigation and subsequent judicial process.