Image copyright AFP Image caption Rights groups called the move a major reversal on human rights

Malaysia's Prime Minister Najib Razak has gone back on a pledge to repeal a controversial sedition law, saying it will instead be strengthened.

Under the law, speech deemed to incite unrest or religious or social tensions or which criticises traditional rulers can carry a three-year jail term.

Critics say the colonial-era law is used to silence political opponents.

Mr Najib had said in 2012 that he would abolish the act, amid raft of political reforms.

But speaking at his party's annual congress on Thursday he said the law would be retained.

Image copyright AFP Image caption The controversial law has its critics both inside and outside Malaysia

"As prime minister, I have decided that the Sedition Act will be maintained," Mr Najib told delegates from his ruling party United Malays National Organisation (UMNO).

He added that the law will "be strengthened and made more effective", with "a special clause to protect the sanctity of Islam, while other religions also cannot be insulted".

A second clause will make it illegal to call for the breakaway of the states of Sabah and Sarawak, on Malaysian Borneo, he said.

Analysis: Jennifer Pak, BBC News, Kuala Lumpur

Prior to the last general election, Prime Minister Najib Razak announced that he would repeal the Sedition Act in July 2012 because it "represents a bygone era" and was part of his reforms to develop Malaysia into a progressive democracy.

Mr Najib said the sedition law would be replaced with a National Harmony Act.

Over the last year a string of sedition charges sparked fears that the prime minister would backtrack on his promise.

Since his coalition won a slimmer majority in the last vote, some analysts and members within Mr Najib's Malay-Muslim based party told the BBC that the prime minister was facing mounting opposition to his reforms from hardliners within his coalition who are pushing for more favourable policies over other races and religion.

Why the controversy of Malaysia's sedition law?

'Rights abusing strategy'

Dozens of people have been targeted under the sedition laws, including opposition leader Anwar Ibrahim and other opposition politicians.

"This is the start of authoritarian rule by Najib," Mr Anwar told AFP news agency.

"They will use the sedition law to intimidate the legitimate voice of the opposition."

Rights group have also criticised Mr Najib and accused the government of suppressing their critics.

"By endorsing the Sedition Act, Prime Minister Najib is doubling down on his bet that a rights abusing strategy is the best way to maintain power," said Phil Robertson, deputy director in Asia at Human Rights Watch.

"This is a major reversal on human rights that will seriously degrade Malaysia's already shaky reputation in the international community," he added.

UMNO has been the dominant party in the ruling coalition Barisan Nasional since independence in 1957.

But public opinion has turned against them in recent years after allegations of corruption and abuse of power.