Mining giant Anglo American on Friday rejected a takeover bid from rival BHP Group, saying the offer “significantly undervalues” the company and its future prospects.

Australia-based BHP on Thursday said it had made an all-share takeover offer which valued the smaller company at £31.1 billion ($38.9 billion). The takeover would have created the world’s largest mining company, according to a Reuters analysis.

Shares of Anglo American were trading down 0.5% at 9:00 a.m. London time, while other mining stocks rose.

In a statement, the British miner said that board members had unanimously rejected BHP’s “unsolicited, non-binding and highly conditional” proposal.

Anglo American’s Chairman Stuart Chambers dismissed the bid as “opportunistic.”

“The BHP proposal is opportunistic and fails to value Anglo American’s prospects, while significantly diluting the relative value upside participation of Anglo American’s shareholders relative to BHP’s shareholders,” he said.

BHP did not immediately respond to a CNBC request for comment.

The offer had included a requirement for Anglo American to demerge its entire shareholdings in South Africa-based Anglo American Platinum Limited and Kumba Iron Ore Limited, two entities which together account for a sizeable proportion of the company’s copper production.

Anglo American Chairman Stuart Chambers said the proposed restructure was “highly unattractive, creating substantial uncertainty and execution risk borne almost entirely by Anglo American, its shareholders and its other stakeholders.”

Shares of Anglo American Platinum rose more than 2% on the announcement, while Kumba Iron Ore moved 0.9% lower.

Mining firms are seeking to shore up copper supplies over the years ahead due to projected shortages and the metal’s key role in the energy transition, with uses in electric vehicles, power grids and wind turbines.

Chambers said that, in its current state, Anglo American was “well positioned” to benefit from that energy transition.

“With copper representing 30% of Anglo American’s total production, and with the benefit of well-sequenced and value-accretive growth options in copper and other structurally attractive products, the Board believes that Anglo American’s shareholders stand to benefit from what we expect to be significant value appreciation as the full impact of those trends materialises,” he said.

— CNBC’s Jenni Reid contributed to this article.