The new deadline is 5 p.m. Eastern Time on Feb. 27.
First Quantum has also relaxed one of the offer's conditions, giving the Vancouver-based company more flexibility to go ahead with the deal even if denied access to certain information.
Inmet has said it's seeking alternatives to First Quantum's $5.1-billion offer.
First Quantum announced Friday that it has received a favourable ruling from the federal competition commissioner on Feb. 7, meeting one of the conditions.
The company has also changed another condition to give First Quantum the discretion to go ahead with a purchase even if it isn't given access to the same non-public information provided to other potential buyers, according to a regulatory filing obtained through DisclosureNet.com.
Philip Pascall, First CEO and Chairman of First Quantum, said in a statement Friday that the deadline is being extended to provide more time for some regulatory approvals that are necessary conditions of the deal.
"We continue to work towards ensuring that we will be in a position to satisfy all the conditions of our offer as soon as possible," Pascall said.
"We have received strong support and encouragement for the offer from Inmet's shareholders," added. "We wish to thank those Inmet shareholders for their support. We encourage them to continue with these efforts, as we continue with our own efforts to engage Inmet's directors and management team in an open dialogue and exchange of information."