Qatar Holding, the second-largest shareholder in takeover target Xstrata, has added to its stake in the miner.

It is its first purchase since it made an unexpected demand for better terms from suitor Glencore last month.

Qatar spent just under £5m to buy an extra 590,390 shares on Friday at 8.47 pounds each, according to a regulatory filing today, taking its stake to 10.997% of the miner and building what is already the largest holding after commodities trader Glencore's 34%.

Glencore, aiming to build a mining and trading powerhouse by tying up with Xstrata, is offering 2.8 new shares for every Xstrata share in a $26 billion offer.

Qatar, which has largely built its stake since a takeover bid was announced in February, was expected to support Glencore but surprised the market in June by demanding an improved ratio of 3.25.

Glencore and Qatar are currently locked in talks.

Xstrata shareholders are due to vote on the deal on September 7, meaning, under UK rules, that Glencore has until around August 24 if it is to change the terms of the bid.

It could do so later, but would have to reschedule the vote for a second time.