* BHP needs shareholder ok if bid rises by more than 22 pct
* UK listing rules measure value of bid vs market value
By Eric Onstad and Michael Erman
LONDON/NEW YORK, Aug 24 (Reuters) - BHP Billiton's hostile $39 billion bid for fertiliser group Potash Corp faces a potential obstacle -- formal approval by its own shareholders -- if it sweetens its offer by more than 22 percent.
That threshold based on UK listing requirements is likely to come into any BHP deliberations on how much to sweeten its bid for Potash Corp, the world's biggest fertiliser company.
"Clearly it's easier for the company not to have go through that whole process, BHP would want to do things as quickly as possible," said analyst Charles Kernot at Evolution Securities in London.
Under the UK listing rules, no shareholder vote is needed at the current $130 per share offer since the total takeover price does not equal to more than 25 percent of BHP's total market capitalisation.
The market value is based on the share price the day before the world's top miner BHP unveiled its offer, totalling about $188 billion.
The requirement would kick in if BHP increased its bid to about $158.50 per share or $47 billion, according to Reuters calculations.
A Reuters poll showed Potash investors think BHP could succeed with a higher bid of $162 a share, while many analysts regard $157 as a winning offer.
"Putting yourself in the Never Never Land of a shareholder vote? That's M&A 101 -- don't do it," said an investment banker not working on the deal.
Analysts and investors say BHP Chief Executive Marius Kloppers has a history of being cautious on M&A deals after he walked away from a hostile bid for Rio Tinto in 2008 during the global economic downturn.
"The board probably would prefer to avoid a shareholder vote, but even if a vote is required it is likely that shareholder approval would be given," said BMO Capital Markets analyst Tony Robson.
SHAREHOLDER ENGAGEMENT
The issue of shareholder involvement in takeover processes is a hot topic in Britain.
Kraft Foods Inc's takeover of Cadbury, and the failure of Prudential Plc's bid for American International Group Inc's Asian arm, have helped fuel debates in Britain about shareholder engagement.
Prudential's $35 billion bid collapsed in June, scuppered by shareholder opposition to a deal that required their support because it entailed a $21 billion rights issue. Some hailed that as a new era of shareholder activism.
In the wake of the Cadbury acquisition, Britain's takeover watchdog started a consultation on a slew of potential changes to the UK's merger rules, which could include raising the minimum acceptance threshold for a bid from 50 percent plus one share.
The Institute of Directors (IoD), a lobby group representing 45,000 senior business people, has argued that any takeover of a large listed UK company should require a vote by shareholders of the acquiring company.
UK listing requirements provide four tests with a 25 percent threshold to determine whether a company needs shareholder approval for a takeover, including the one measuring the total value of the transaction.
According to analysts, only one other test was close to the threshold -- the gross capital test which measures the enterprise value of the target company versus the acquirer.
The other two tests measuring gross assets and profits are well below 25 percent, analysts added. (Additional reporting by Quentin Webb in London; Editing by David Holmes)