(Adds energy policy context and analyst comment, paragraphs 4-6)
By Marco Aquino
LIMA, March 26 (Reuters) - State energy firm Petroperu on Thursday pulled out of plans to acquire a 25 percent stake in two oil blocks that it had planned to develop with conglomerate Grana y Montero, Petroperu said in a statement.
The reversal marks a shift in President Ollanta Humala's ambitions for Petroperu to take part in oil production for the first time in more than 20 years.
Leaders from the Petroperu workers union said on Wednesday that the company's board was under pressure from the government to drop shares in the concession.
Oil industry insiders based in Peru said Humala's energy strategy had been tainted by erratic policymaking during his less-than four years in power. In that time, he has named four Petroperu presidents including a new one last week, and also just appointed a new energy minister.
"This bodes ill. What we see is a lack of consistency in energy policymaking," said Aurelio Ochoa, a former head of the industry regulator earlier in Humala's presidency.
Ahead of Petroperu's announcement, union leaders said workers would hold an indefinite strike if the firm back-tracked on its plans. They said the government wanted to prioritize a $3.5 billion upgrade of the Talara refinery.
Petroperu's board of directors agreed in February to take up its option of a maximum 25 percent share in blocks 3 and 4, which Grana y Montero won the rights to develop in December.
But on Thursday the board said the decision had not complied with Petroperu's own legal requirements.
"The board of Petroperu agreed not to approve the company's participation as a non-operating partner in the licence agreements for the exploitation of hydrocarbons in blocks 3 and 4," Petroperu said in the statement to the industry regulator.
Grana y Montero will take over operation of the blocks in April once the contract of the current operator Upland Oil and Gas expires. The conglomerate has pledged to invest $600 million over a 10-year period to develop the concessions.
Under a law introduced during Humala's presidency, Petroperu has an option to take control of up to 25 percent of oil concessions that come up for auction, subject to regulator approval.
The reform was part of Humala's drive to raise Petroperu's profile and transform the state company into a bigger regional player along the lines of Brazil's Petroleo Brasileiro Petrobras or Colombia's Ecopetrol. (Writing by Richard Lough; Editing by Bernard Orr and Grant McCool)