* Three state coal mines to close by 2018
* Coal-fired power holding refuses to pay agreed price
BUCHAREST, July 19 (Reuters) - About 1,100 miners refused to leave the underground tunnels at three old coal mines in Romania's mountainous Jiu valley on Friday to protest a state holding company's refusal to honour an agreement on price.
Last year, Romania decided to close the valley's state-owned hard coal mines of Petrila, Paroseni and Uricani in stages by 2018, and merge its remaining four viable mines with two coal-fired power producers to create integrated holding Hunedoara.
The European Union backed the government plan to spend about 1.2 billion lei ($354.3 million) to close the three mines, part of wider reform plans for the outdated, largely state-owned power sector in its second-poorest member.
But Hunedoara refused to receive hard coal from the three mines at the price agreed with the EU under the mine closure programme, the economy ministry said on Friday.
Miners refused to leave the tunnels on Thursday, and have demanded that leftist Prime Minister Victor Ponta join them in Jiu valley to help solve the problem.
"People say they will not leave the underground until the terms of the 2018 closure programme are guaranteed," union leader Laszlo Domokos was quoted by state news agency Agerpres as saying. "Otherwise, the protest ... will continue and they are not ruling out going on hunger strike."
Economy ministry officials were negotiating with the miners on Friday.
Miners' unions were a potent political force in the early 1990s, when the state-owned industry employed almost half a million people, but their influence has withered under a series of largescale layoffs that have fuelled unemployment, poverty and environmental degradation in Jiu valley.
Coal powers more than 40 percent of Romania's power plants, but most of it is lignite, which is softer than hard coal and is dug in open pits. ($1 = 3.3869 Romanian lei) (Reporting by Ioana Patran; Editing by Luiza Ilie and Elizabeth Piper)