(Reuters) - Anglo American was pressed by key shareholders including BlackRock ( BLK ) to extend talks with BHP Group ( BHP ) over its proposed 38.6 billion pound ($49.18 billion) mining merger, the Financial Times reported on Saturday.

BHP, the world's biggest listed mining group, now has until May 29 to make a firm bid for Anglo American or it will be forced to walk away for at least six months under the UK's takeover rules after it was granted a one-week extension on Wednesday.

BlackRock ( BLK ) was among a handful of investors that encouraged meaningful negotiations with BHP, the FT said, citing people close to the situation.

Two other significant shareholders, Ninety One and Sanlam Investments, also backed the decision to extend talks, despite concerns about a deal structure that requires Anglo to spin off its stakes in its South African platinum and iron ore units, the newspaper added.

U.S.-based asset manager BlackRock ( BLK ) owns a 9.6% stake in Anglo, according to LSEG data, and is also a BHP shareholder.

BHP will stand firm on the structure and value of its latest takeover proposal, focusing instead on allaying its target's concerns around execution risks over the coming week, Reuters reported on Thursday.

The FT said that according to people familiar with BHP's thinking there was only scope for "smaller, creative structures to better share the risks".

However, people close to Anglo cited by the newspaper said the structure needs altering or BHP must pay more.

Anglo American, BHP Group ( BHP ) and Blackrock ( BLK ) did not immediately respond to requests for comment.

($1 = 0.7849 pounds)