The soon-to-be merged groups already agreed on a name, Glencore Xstrata International, and executives have settled their corporate positions and roles but the planned marriage appeared to have overlooked an important detail - the expressed consent of independent investors.

Glencore and Xstrata officials have indicated earlier this week that the two firms' merger of equals will be finalised this week but two large shareholders were not too keen on expediting the whole process, media reports said.

Standard Life and Schroders, which together wields 3.6 percent of Xstrata shares, said in separate statements on Tuesday that the deal will grossly undervalue their shares.

Already owning 34 percent of Xstrata shares, Glencore has proposed to scoop up the remaining 66 percent for $US41 billion.

The offer, however, fell short of Xstrata shares real market value, according to Standard Life equities chief David Cumming, who stressed that "the proposed exchange ratio clearly undervalues Xstrata's assets and future earnings contribution."

"Consequently it is our intention to vote against the deal unless the merger terms for Xstrata shareholders are materially improved," Cumming told the UK-based The Guardian.

The same sentiment was aired the same day by his Schroder counterpart, Richard Buxton, who characterised the proposal as not compelling enough.

"I'm in complete agreement with Standard Life and we intend to do exactly the same. This is a fabulous deal for Glencore, it's probably a great deal for the Xstrata management, but it's a poor deal for Xstrata's majority shareholders," Reuters reported the Schroder fund manager as saying.

"If they keep describing it as a merger of equals, why don't Xstrata shareholders get 50 per cent? We continue to think Xstrata's assets and growth profile are superior to Glencore," Buxton added.

For the deal to push through, Xstrata needs to convince 75 percent of its shareholders to green light the agreement but analysts said that the point raised by Standard Life and Schroder could pick up more shareholders' support and snowball into a major opposing force against the merger.

According to Reuters, "only 16 percent of Xstrata's register have to vote against the deal to block it, which means there is a significant risk Glencore's proposal isn't passed," quoting Broker Liberum Capital in a note it released to investors.

Prior to the emerging opposition, the deal's more finer details have been ironed out, The Guardian wrote, with both camp agreeing to adopt Glencore's corporate set up and picking out London as its listing base.

It is likely too that Switzerland will remain as the new entity's HQ since both Xstrata and Glencore call the country its home base.

Glencore chief Ivan Glasenberg also reportedly gave way to Xstrata CEO Mick Davis to head the new giant company, with most of Xstrata's key executives also poised to assume the top positions in running the merged group.

Glasenberg, The Guardian said, will become the new firm's President and second-in-command of Davis but analysts noted that the former HSBC boss will also enjoy the distinction as the biggest single shareholder of Glencore Xstrata Intl.