Maltese enter a voting site in Valetta to cast their ballots on the controversial referendum. [AFP]

Citizens of Malta are voting in a referendum on whether or not to legalise divorce.

The referendum asks the Mediterranean island's nearly 400,000 mainly Catholic voters whether parliament should introduce a new law that would allow couples to obtain a divorce after four years of separation.

A pre-poll opinion survey also found around 40 per cent of the electorate undecided over the issue.

By early afternoon, just over 34 per cent of eligible voters turned up to cast their ballots, government officials told the AFP news agency.

Jeffrey Pullicino Orlando, a member of the ruling centre-right Nationalist Party, shocked his own party in July last year when he presented a bill in parliament for the introduction of a conservative form of divorce.

Lawrence Gonzi, the country's prime minister, who is strongly against divorce but lacks room to manoeuvre because his government only has a one-seat majority in parliament, eventually decided that the people's judgement on the matter must be decided in a referendum.

Opposition leader Joseph Muscat has said he is strongly in favour of divorce, even if he is "the last man standing".

"The divorce referendum is a vote for modernity and an opportunity for those whose marriage has broken down to be able to start afresh," Muscat said on Thursday.

For Gonzi, however, divorce offers "no solutions" for society.

"We have to help people whose marriage has broken down, but we also have to help couples prepare better for marriage so that marriages do not break down and the value of an indissoluble marriage is bequeathed to the young," Gonzi said.

The vote is seen as a test of the influence of the Roman Catholic Church in a country where at least 72 per cent of people consider themselves consistent observers of the faith.