The priceless paintings stolen from the private collection of Judge Giovanni Bonello could suffer irreparable damage if mishandled or badly stored, art expert Fr Marius Zerafa said.

Expressing fear that the works of art may have already left the country, Fr Zerafa, former director of museums, said the theft was a great loss to the art world and more especially to Judge Bonello, whose collection has been completely disrupted.

"(Judge Bonello) has a very good selection, collected over more than 80 years when his father Vincenzo was curator of the Museum of Fine Arts, and he is an avid collector himself," he said.

Fr Zerafa said that among the collection, Judge Bonello was known to have paintings attributed to Caravaggio, as well as to Mattia Preti and Giuseppe Calì, among other masters.

A Caravaggio painting, the Nativity with St Francis and St Lawrence, which remains one of Italy's most wanted pieces of stolen art, is valued at more than Lm13 million.

The police are not releasing details of the haul at this stage of the investigations and are in contact with Interpol.

The theft of about 20 priceless paintings was reported by Judge Bonello's sister, Anna Xuereb, on Tuesday morning after she discovered the break-in during one of her visits to their late father's residence in Old Bakery Street, Valletta.

The thief or thieves managed to get into the building by first pulling up the metal grating from the pavement, which gave them access to a small cellar situated right underneath the entrance hall of the house.

It is understood that the thieves used jacks to support the overhead roof while they worked to dislodge one of the slabs to work their way into the house.

Sources said one of the biggest hurdles the police faced in their investigations was the fact that they cannot establish the exact day the robbery took place because Mrs Xuereb said she had not visited the residence for the past two or three weeks.

The sources said it could not be ruled out that the paintings had already been smuggled out of the country.

The thieves had a fine taste for art and all evidence shows that the robbery was pre-planned and the paintings stolen to order. Other valuables, which were clearly within reach, remained untouched.

Meanwhile, Fr Zerafa believes that unless the theft was commissioned and the paintings pre-ordered by an art connoisseur, the thieves would go underground for a few years until they feel it is safe to sell them.

Fr Zerafa knows from experience. He was personally involved in the retrieval of the Caravaggio's St Jerome, which was stolen from St John's Co-Cathedral in the 1980s.

Two years after it was stolen, he received a letter which included a photo of the St Jerome - the thieves were demanding a ransom.

Fr Zerafa bought time, worked with the different ministries, and sleuthed on his own. More than a year later the St Jerome was found in a local factory, rolled up in a trunk.

"The painting was damaged because it had been rolled up and its paint cracked," he said.

It was sent to Rome to be restored and was re-hung in the cathedral.

"It is hard to say what will happen. We may not hear anything about these stolen masterpieces for years. We just have to wait for the thieves to make a false move and in the meantime, keep calm, and pray."