* Demerger in best interest of shareholders - independent expert

* Billionaire Cannon-Brookes' stake heightens vote risk

* Demerger vote set for June 15

* AGL shares outperform battered market (Adds AGL chairman, CEO comments throughout)

By Indranil Sarkar and Sonali Paul

May 6 (Reuters) - AGL Energy's plan to split won backing from an independent expert, but the company's top officials on Friday said they will need to work hard to secure shareholder support against a billionaire raider looking to block the demerger.

Tech billionaire and climate activist Mike Cannon-Brookes, thwarted in a A$5.4 billion ($3.8 billion) joint takeover bid in March, this week snared an 11% stake in AGL, injecting uncertainty into the demerger vote, set for June 15, which needs 75% support from votes cast.

One pension fund voiced opposition to the demerger after Cannon-Brookes made his move, but others said they were waiting to see the demerger scheme booklet, released on Friday.

"I think the jury's very much still out on this," AGL Chairman Peter Botten told Reuters, referring to the outcome of the vote.

Advisory firm Grant Samuel, engaged by AGL to prepare an independent expert report, said "the demerger is a positive solution to the challenges facing AGL Energy and has the potential to create long-term value for shareholders".

AGL's directors unanimously recommended voting in favour of the demerger.

AGL wants to split itself in two, after suffering a 75% slump in its market value over the past five years. The company's shares slipped 0.8% on Friday, but held up better than the broader market.

The proposed demerger would create two separate ASX-listed companies — an energy retailer AGL Australia and a coal-fired power producer Accel Energy.

"There is a clear case that the status quo is sub-optimal and change is required," the report said, adding that options like early closure of its coal plants, as Cannon-Brookes wants, or selling the company had significant drawbacks.

With Cannon-Brookes holding 11.28% of the shares, only a further 14% would need to oppose the split for the demerger plan to be blocked - but actually less than that given AGL has a big base of small shareholders, who typically don't vote.

Botten said Cannon-Brookes' plan to keep the company together would destroy shareholder value far more than alternatives AGL considered.

"It's the absolute worst case scenario," he said.

For shareholder value, power affordability and grid reliability, it made no sense to shut AGL's coal plants early because they are the lowest cost plants in the market, CEO Graeme Hunt said.

($1 = 1.4079 Australian dollars) (Reporting by Indranil Sarkar, Savyata Mishra in Bengaluru, Sonali Paul in Melbourne; Editing by Subhranshu Sahu, Uttaresh.V and Emelia Sithole-Matarise)