Prime Minister Joseph Muscat's pledge to resign after the election of a new leader on January 12 failed to appease critics who have been calling for his immediate resignation, and who kept up the pressure after his announcement on Sunday. 

Daphne Caruana Galizia's son Matthew posted a brief reaction on Twitter, promising that "people will be out in the streets again tomorrow".

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."

Civil society group Repubblika, which has led sustained protests against the Prime Minister over the last two weeks, said the delayed resignation represented "a threat and an insult to the process of justice that Joseph Muscat has been perverting for the past three and a half years".

The group said Dr Muscat was at the heart of the largest scandal in Maltese political history and should resign immediately to face investigation and for evidence to be preserved.

"There is a reasonable and serious suspicion that at the very least, Joseph Muscat was, and still is involved in a cover up to free his friends from being charged with the assassination of Daphne Caruana Galizia," Repubblika said.

"Due to this suspicion, there exists another reasonable suspicion that Joseph Muscat has to account for further direct and serious responsibilities in this matter."

Andre Callus from Moviment Graffitti, which has also participated and organised recent protests, said the announcement meant that, regardless of the investigations' outcomes, there would always remain the suspicion that those in power had interfered to ensure the truth did not emerge. 

He said the Prime Minister had not even acknowledged that his resignation was a result of recent events, but presented it as a natural and planned event.

"Joseph Muscat should have, at the least, shouldered responsibility for his decision to retain [former chief of staff] Keith Schembri - whose illicit relationship with Yorgen Fenech through 17 Black is a fact not an allegation - and resigned immediately," Mr Callus said.

National Book Council chairman Mark Camilleri, a Labour Party exponent who has been a vocal critic of the government's handling of recent developments, said the decision had created "a 42-day window of authoritarian rule as approved by the majority of Labour MPs".

"At the same time the mafia will re-entrench and we may lose the chance to decapitate it," he added.

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.

"Any government has a structure that can, and should, be able to withstand the resignation of a prime minister. If the Prime Minister himself cannot fully appreciate or understand the importance of this, in the national interest, then it is incumbent upon his colleagues in cabinet and in Parliament to withdraw their support and to act in the national interest."

Nationalist MEP Roberta Metsola described the Prime Minister as "petty, petulant and irresponsible to the end" and his decision to stay on for a further six weeks as "despicable".

"He wants to plunge Malta into six more weeks of crisis as he clings on by his fingernails to try and save himself and his friends," she said. 

In another tweet she added: "How many documents can one shred in 42 days?"