Indonesian group takes on Nat Rothschild with proposed bid for ARMS

* Firm offer to be on the table within two days - adviser

* Rothschild, some shareholders sceptical about the bid

* Proposal comes ahead of vote on restructuring plan (Updates with quotes from Rothschild, shareholder, adviser)

By Silvia Antonioli and Cindy Silviana

LONDON/JAKARTA, April 15 (Reuters) - A potential bid by Indonesia's Sinarmas Group and hedge fund Argyle Street Management for London-listed Asia Resource Minerals Plc (LSE: ARMS.L - news) (ARMS) risks scuppering a long-awaited restructuring backed by ARMS co-founder Nathaniel Rothschild.

Shares (Berlin: DI6.BE - news) in coal miner ARMS, formerly known as Bumi, jumped more than 90 percent to as high as 29 pence on Wednesday after Asia Coal Ventures, a vehicle controlled by Argyle and funded by tycoon Era Tjipta Widjaja's Sinarmas Group, said on Tuesday it was considering a 210 million pound ($310 million) cash offer.

The potential offer of 41 pence per share comes a week ahead of a vote on a vital refinancing and recapitalisation plan which could see Rothschild, a scion of the banking dynasty and ARMS' No.3 shareholder, increase his stake of about 18 percent.

Argyle is also a shareholder in ARMS, whose business is focused in Indonesia, with a 5 percent stake.

Some shareholders were sceptical about the potential offer.

"This may be an attempt to muddy the waters rather than anything. Until there is an actual bid from a credible counterparty there is nothing for us to take a view on," said Cato Stonex, a co-founder of UK-based asset management firm Taube Hodson Stonex, which has about 1 percent in ARMS.

Stonex said his current intention was to vote in favour of the plan proposed by the company and Rothschild.

An adviser close to the matter, however, said a firm offer should be on the table within two days.

"Nat (Rothschild) is on the cusp of taking control of the company on the cheap, without paying a control premium. But now there is an alternative," he said. "People who think this is a flaky proposal will be proven wrong within 48 hours."

Argyle's discontent with the proposed solutions to problems at ARMS, which has been hit by executive battles and tumbling coal prices, have led it to look for an alternative together with an experienced Indonesian partner, it said.

Sinarmas Group, whose businesses include energy, palm oil, property, pulp and paper, said the acquisition would help support its own power business.

Sinarmas has a Jakarta-listed coal mining unit, PT Golden Energy Mines Tbk, and owns some power stations.

"As a national company, we support the national priority, especially infrastructure projects," Sinarmas Group Managing Director Gandi Sulistyo told Reuters.

Indonesian President Joko Widodo, who took office last October, has identified developing the country's creaking infrastructure as a focus for his administration.

"This still does not explain why Sinarmas' proposed bid is via an obscure shell company managed by a hedge fund. It cannot possibly derive any synergies given that PT Golden Energy is not making the bid directly," Rothschild told Reuters.

"Furthermore, comments about ARMS needing to be managed by a national interest are not valid -- (we) have already met with Joko Widodo in March to discuss the investment programme in the power sector via ARMS and (its subsidiary) Berau."

ARMS shares were trading at about 25 pence by 1500 GMT. (Additional reporting by Fransiska Nangoy; Writing by Eveline Danubrata; Editing by Clarence Fernandez, Gopakumar Warrier and Mark Potter)