The EU has warned Malta against any "political interference" in the investigation of the 2017 murder of journalist Daphne Caruana Galizia.

In a phone call with Justice Minister Owen Bonnici, European Commission Vice President Vera Jourova "insisted that the investigation has to be brought to conclusion without any political interference," a spokesman said on Tuesday.

She also "expressed her concern regarding the situation in Malta," the spokesman said.

Caruana Galizia's family on Monday called for police to investigate Muscat amid allegations he obstructed justice by protecting key figures in the case, including his chief of staff.

Jourova, who oversees rule of law issues for the EU member states, underlined in the call that "more work needs to be done" in Malta on maintaining an independent legal system in the country. 

A European Parliament delegation had talks with Dr Muscat and Dr Bonnici on Tuesday morning and is also due to meet the police commissioner.

Dutch MEP Sophie in't Veld said after meeting Dr Muscat that she was 'unconviced' by what the delegation had been told.

“We can talk for a long time about the details of the case but ultimately this is about truth and credibility of the highest political office here," she told reporters.

Last month, Vera Jourova, who is responsible for rule of law within the European Commission, told MEPs that she would be keeping her promise to “look very closely” at the investigation into Daphne Caruana Galizia’s murder.

“I met Daphne Caruana Galizia’s family, I gave a promise to her mother that I will look very closely at the investigations. I made a promise to her son to do my best to protect journalists in Europe. These promises are very strong commitments. I am happy to do more to keep these promises,” she said.