Romania's ruling left-wing party withdrew its support for Prime Minister Sorin Grindeanu on Wednesday, a move likely to bring down his government, which has only been in power since January.

It is the second major crisis to rock the government since it won a thumping poll victory in December, barely a year after being forced from office over a deadly nightclub blaze.

"A prime minister is only legitimate as long as he has the support of the parties that voted for him. We have withdrawn that support," Liviu Dragnea, the leader of the Social Democrats (PSD), said after cabinet ministers resigned en masse on Wednesday evening.

Grindeanu however refused to step down, saying he would only quit once centre-right President Klaus Iohannis had appointed his successor from the PSD.

"I will not resign, I am a responsible person," the 43-year-old told journalists in Bucharest.

His statement appeared to contradict Dragnea, who said the premier had agreed to leave.

The PSD, holder of a parliamentary majority with the small ALDE party, accused Grindeanu of failing to implement economic reforms in the European Union's second-poorest country after Bulgaria.

But some observers say the move is the result of a fallout between the prime minister and Dragnea.

The powerful party boss, 54, was barred from running for premier because of a voter fraud conviction and is currently on trial for alleged abuse of power, a charge he denies.

The surprise development comes after mass protests forced the PSD to scrap a controversial decree aimed at watering down anti-corruption laws in February.

Analysts say the internal disagreement may have been linked to the failed proposal, which could have allowed Dragnea to run despite his conviction.

"Liviu Dragnea only wants one thing -- amendments to the anti-corruption laws" that currently prevent him from becoming premier, according to former PSD member Alin Teodorescu.

The government had already got off to a rocky start when President Iohannis rejected the PSD's first prime ministerial candidate, who would have been the country's first female and Muslim head of government.

The crisis eventually ended with Grindeanu's nomination in January.

Dragnea has made no secret of the fact he had chosen a candidate close to him.

Grindeanu, a former communications minister, was seen as a "disciplined soldier" within the PSD ranks, having joined the party at a very young.

"I wanted a man I could trust, a man who wouldn't use his government position as a springboard," Dragnea said after Grindeau's appointment.

The PSD quit following angry protests in 2015 over a nightclub fire in Bucharest that killed 64 people.

The inferno was blamed on corrupt officials turning a blind eye to fire rules.

A decade after joining the European Union, the nation of 20 million inhabitants remains mired in poverty.

Nonetheless, the former communist state logged first-quarter economic growth of 5.7 percent, according to data released last month -- the fastest rate in the EU.