A key Israeli opposition party has backed a unity government that would bring to an end Benjamin Netanyahu's time as the country's longest serving prime minister.

The ultra-nationalist leader Naftali Bennett said his party would join talks to form a governing coalition with centrist party leader Yair Lapid.

Mr Lapid has until Wednesday to announce the result of his efforts.

Mr Netanyahu, meanwhile, said the proposed deal would "weaken Israel".

The prime minister, who is on trial for fraud, fell short of a decisive majority at a general election in March. It was the country's fourth inconclusive vote in two years - and again Mr Netanyahu failed to secure coalition allies.

Mr Bennett, who leads the Yamina party, made his announcement in a televised address.

"I will do everything to form a national unity government with my friend Yair Lapid," he said.

The proposed coalition would bring together factions from the right, the left and the centre of Israeli politics. While the parties have little in common politically, they are united in their desire to see Mr Netanyahu's time in office come to an end.

Speaking shortly after Mr Bennett's announcement, the prime minister accused Mr Bennett of carrying out "the fraud of the century" - a reference to the Yamina leader's previous public promises not to join forces with Lapid.

Mr Netanyahu said the former defence minister's approach would be "a danger for the security of Israel". He did not elaborate.

Mr Lapid, a former finance minister, was given until 2 June to form a new coalition government after Mr Netanyahu failed to do so. His Yesh Atid party came second to Mr Netanyahu's right-wing Likud at the last election.

Mr Bennett's party holds a crucial six seats in the 120-member parliament that would give the proposed opposition coalition a clear majority.

On Saturday night, Mr Netanyahu's Likud party made an offer to Mr Bennett and the leader of another potential coalition party to share the premiership in a three-way split.

However, his offer was rejected. But the prime minister repeated the same option on Sunday.

Under Israel's electoral system of proportional representation, it is difficult for a single party to gain enough seats to form a government outright. Smaller parties are usually needed to make up the numbers needed for a coalition.