Romanian President Klaus Iohannis has rejected the prime ministerial candidate nominated by the left-of-centre Social Democrats.
Sevil Shhaideh would have been Romania's first female and first Muslim prime minister. President Iohannis has given no reasons for his decision.
But Ms Shhaideh has been criticised for lacking political experience, only serving once as a regional minister.
The Social Democrats (PSD) won the vote, and hope to form a coalition.
"I have properly analysed the arguments for and against and I have decided not to accept this proposal," Mr Iohannis said in a televised statement.
"I call on the.... coalition to make another proposal," he said.
The president did not explain the factors he considered in making his decision but there have been allegations that Ms Shhaideh would be PSD leader Liviu Dragnea's "puppet" if she became prime minister.
The PSD nominated her after its resounding election win on 11 December when its pledges to raise pensions and implement tax cuts secured it about 45% of the vote.
Mr Dragnea was forced to withdraw his bid to become prime minister because of his conviction for election fraud, for which he received a two-year suspended jail sentence.
The PSD's election win comes after public outrage over a nightclub fire in November 2015 which killed 64 people and triggered the resignation of Prime Minister Victor Ponta.
Many Romanians saw the fire, at Colectiv club in Bucharest, as the tipping point.
The tragedy prompted a nationwide attempt to clean the country up.
Sevil Shhaideh
A member of Romania's tiny Turkish minority, she is seen as a political newcomer
Most of her working life has been spent as an economist and a civil servant
Widely regarded to be politically inexperienced, she has served only a six-month stint as development minister
Seen as being personally close to Mr Dragnea, who is reported to have been a witness at her wedding
