Israeli Prime Minister Benjamin Netanyahu cast his vote on Tuesday in his Likud party's primary, which he is expected to easily win, besting his sole competitor, the hardline settler Moshe Feiglin.

Netanyahu and his wife Sarah were first in line to cast their ballots in the vote, which the premier decided in December to move forward.

The surprise decision sparked speculation about whether he would also bring forward general elections, currently scheduled for around November 2013.

Netanyahu said the move was a way to save money, saying holding the primary on the same day as the party's convention would save millions of Israeli shekels.

But commentators characterised it as an attempt to capitalise on his domestic popularity, with polls placing him far ahead of Feiglin as well as future competitors for the premiership.

His Likud party also remains well ahead of the opposition, according to recent polls.

Some 125,000 people who have been registered as Likud members for more than 16 months are eligible to cast a vote at one of 150 stations across the country, including in the West Bank, where Feiglin cast his ballot.

Voting started at 8am (GMT) and with polls open until 8pm (GMT), first results are not expected until around 10pm (GMT).

In Jerusalem, home to Likud's biggest branch and a Feiglin stronghold, rival tents set up outside a polling station blared music intended to entice voters. Feiglin's tent attracted the largest crowd, most of them religious Jews.

With Netanyahu expected to score upwards of 75 per cent of the vote, as he did during Likud's last primaries in August 2007, the winner of the contest in not in question.

But the process is being closely watched by those curious to see how well Feiglin does.

A religious Jew who lives with his wife and five children in the West Bank settlement of Karnei Shomron, Feiglin is known for his hardline positions, particularly on the Palestinians and Israel's Arab minority.

He opposed the Oslo peace accords and has said that Palestinians and Arab Israelis should be paid to move to Arab countries.

In 2007, he won around 25 per cent of the primary vote, and now hopes to capitalise on settlers' anger at Netanyahu over his plan to comply with a High Court ruling ordering the relocation of a West Bank settlement outpost.

Feiglin has also sought to encourage activists who do not vote for Likud in general elections to become party members in order to influence the primaries, according to political commentator Hannan Crystal.

"Thousands of settlers who don't vote for Likud in the legislative elections, choosing instead groups that are more to the right, have registered with Likud so as to influence the party line," Crystal told AFP.

"They represent about nine percent of its members. But their strength is greater than that because they form a bloc that will vote massively against other members of Likud," he added.

Crystal said a strong showing for Feiglin could push Likud to the right because its members would fear angering his voting bloc as the party puts together its list of candidates for the general election.

Netanyahu's supporters in Jerusalem told AFP this week that they were genuinely concerned about Feiglin's challenge, fearing complacency among the prime minister's supporters could give his opponent an outsize showing.