WARSAW (Reuters) -Poland's prime minister on Thursday ordered regulators to investigate trading in the shares of a small utility which saw a twelvefold increase in value this month before falling sharply.

Trading in shares of heat and electricity provider Elektrocieplownia Bedzin (EC Bedzin) was halted on the Warsaw Stock Exchange on Thursday after they fell more than 35% on the day.

Prior to this the stock had risen 1,200% between June 1 and its Thursday peak, Refinitiv Eikon data showed. The company had offered no explanation for the rally.

As of Wednesday's close it had a market capitalisation of 222 million zlotys ($55.1 million).

"Due to doubts regarding the shares of EC Bedzin, I asked the chairman of (financial regulator) KNF to prepare a report as a matter of urgency, which will be the basis for further steps," Mateusz Morawiecki wrote in a Twitter post.

EC Bedzin could not be reached by telephone on Thursday by Reuters and did not immediately reply to an emailed request for comment.

The crash in the share price on Thursday came after Orlen Synthos Green Energy, a joint venture between Polish refiner PKN Orlen and Synthos Green Energy that plans to deploy small modular nuclear reactors in Poland, said it did not plan to work with EC Bedzin.

The utility said in a statement late on Wednesday that it had signed a non-disclosure agreement with Orlen Synthos Green Energy concerning potential cooperation on the reactors.

Orlen Synthos Green Energy said that the non-disclosure agreement had been signed at the request of EC Bedzin, which wanted to show it a potential site.

KNF spokesperson Jacek Barszczewski said the regulator was investigating the matter.

"The behavior of EC Bedzin's shares, as in any case where there are doubts about the possibility of using confidential information or potential manipulation, is subject to a thorough analysis by the KNF," he wrote in a Twitter post.

The watchdog will use all its tools to investigate the circumstances of the share price move, he added.

($1 = 4.0290 zlotys)

(Reporting by Alan Charlish; Editing by Jan Harvey)