Petaquilla said in a statement Monday that the "hostile" cash and share offer fails to provide adequate value to shareholders and has been unanimously rejected by company's board.
"In particular, the Inmet offer does not adequately reflect Petaquilla's current value or the value of Petaquilla's substantial growth prospects, nor does it take into account the true value of Petaquilla to Inmet and its shareholders," executive chairman Richard Fifer said.
Inmet announced its offer of 48 cents per share in cash — or 0.0109 of an Inmet share for each Petaquilla share — in early September in a bid to consolidate the area around its Cobre Panama project.
Petaquilla's Molejon project neighbours Inmet's US$6.2-billion Cobre Panama copper-gold porphyry project, Panama's largest-ever mine development.
Inmet said it would also allow Petaquilla to proceed with a spin-out of Petaquilla's assets in Spain to Petaquilla shareholders.
News of the bid sent Petaquilla's share price soaring some 60 per cent when it was announced, but the company urged shareholders to take no action while it reviewed the offer.
It now is asking them not to tender to the offer.
In addition to Cobre Panama, Inmet produces copper and zinc at its operations in Turkey, Spain and Finland.
On the Toronto Stock Exchange, Petaquilla shares closed at 63 cents Friday, while Inmet stock closed at $43.15.