Revealed: the other two billionaires with a stake in Mitchells & Butlers

Two more members of the "Sandy Lane set" of Barbados billionaires, which includes veteran currency trader Joe Lewis and horse-racing magnates JP McManus and John Magnier, have secretly built stakes in Mitchells & Butlers, the Observer has learned.

It is a extraordinary twist to the boardroom wrangle at the listed pub company, which owns the All Bar One and Harvester chains.

Michael Tabor and Derrick Smith – both with backgrounds in betting shops but now powerful investors and big names in the world of horse racing – have separately used nominees and front companies to buy substantial blocks of shares in M&B. The pub company used provisions under the Companies Act to discover the identities of the two men.

The holdings will add to the pressure on M&B, and its chairman Simon Laffin, who faces being ejected at the group's annual meeting next month. The appearance of the two men on the M&B shareholders' register comes amid a remarkable boardroom bust-up at the company, which has unfolded over the past two weeks, as M&B has locked horns with its two biggest shareholders.

The pub group has called in the takeover panel to investigate whether "a number of shareholders" are working together to take control of the business without making a bid, and has fired their four boardroom representatives.

It had been assumed that the panel's investigations would focus on M&B's two biggest shareholders. They are Piedmont Inc, controlled by Lewis, which owns 22.3% of M&B, and Elpida Group, which speaks for 17.6% and is controlled by Magnier and McManus. However, it is likely the inquiry will also be looking at the stakes owned by Tabor and Smith.

The five billionaires are regular and long-standing business associates and co-owners of the Sandy Lane hotel in Barbados, where royalty, celebrities and businessmen regularly line up their sunbeds.

Frequent Sandy Lane guests include Michael Winner, Hugh Grant and Joan Collins. Tiger Woods's wedding took place in the hotel's gardens, and every Christmas Topshop tycoon Sir Philip Green descends for a holiday with friend and business partner Simon Cowell.

Derrick Smith is a racehorse owner and Barbados resident who owns one of the largest private homes on the island, next to the Sandy Lane, as well as other property in Florida. Much of his fortune is understood to have come from currency trading alongside Lewis and Tabor. He once worked for Ladbrokes but went into business on his own in 1988. He first gained prominence in racing with co-owner Tabor and has owned a string of big-race winners.

Tabor, who splits his time between homes in Monaco and Barbados, started his career as a bookie. He built a chain of betting offices which he sold to Coral in 1995 and then made another fortune from an investment in the Next Generation chain of fitness clubs. He now has a stake in bookmaker Victor Chandler's internet betting business and co-owns a string of racehorses with Magnier's wife Susan.

Tabor's son is Ashley Tabor, who once worked for Cowell. Last year, backed with £375m cash from investors including Magnier, McManus and his father, he took over GCap Radio, the group behind Capital Radio and Classic FM.

The M&B shareholder register reveals that the group used "section 793 notices" – a power granted under the Companies Act to discover that Tabor and Smith had taken stakes. The notices force entities listed on the register to reveal their beneficial ownership.

One notice shows that 4,885,193 M&B shares – a 1.2% stake – are held by nominees Pictet & Co on behalf of a company called Rainsford Management, whose beneficial owner is Michael Tabor.

A second notice shows that 10,550,848 shares – a 2.6% stake – are held by nominees Lombard Odier on behalf of Smoothfield Holdings, whose beneficial owners are the family of Derrick Smith.

The Observer contacted the spokesman for Lewis, and the representative of Magnier and McManus, but neither returned calls to say whether they were aware of Tabor and Smith's stakes.

The schism in the M&B boardroom burst into the spotlight when it emerged that Richard McGuire, a director on the M&B board who represents Lewis, had blocked the appointment of former Asda boss Archie Norman and then former Hamleys chairman Simon Burke as the group's new chairman. McGuire also objected to the group's senior independent director, Simon Laffin, stepping up to the chair and demanded his resignation, warning him if he did not go, a small number of large shareholders would vote against his election at the shareholder meeting next month.

Exactly what the dispute between the two sides concerns is unclear, but it is thought to centre on a fundamental disagreement about the strategy of M&B, even though the group has been performing relatively well in recent months.

M&B, however, reacted with a night of the long knives, axing McGuire and three other directors it claimed were not independent, installing Laffin as chairman and calling in the takeover panel.

Laffin now faces a battle to remain as chairman. If Lewis, Magnier, McManus, Tabor and Smith were to vote against him, he would need to mobilise almost every other one of M&B's 40,000 shareholders to vote in his favour – a near-impossible task.

Alternatively the takeover panel could intervene. If it decides the shareholders are working together against the interests of other investors, it could order them to launch a full bid – or it could cap their voting power at 30%. However, it is notoriously difficult to prove shareholders are acting in concert. M&B refused to comment.

Other stakes are owned by investors whose loyalties are currently unclear. They include multimillionaire property entrepreneur Trevor Hemmings – who also owns a string of racehorses – and the Italian Martini family, who earlier this year backed an attempt by the Leo hedge fund to secure seats on the M&B board.

Only one institutional investor, Standard Life, has so far thrown its weight behind the boardroom purge, saying Laffin is trying "to ensure that the company is run in the interests of all its shareholders".

In the next week M&B will despatch its annual report to shareholders, which will include details of the board as it was before the bust-up. A covering letter will be sent out to shareholders – to explain the battle that has broken out since the document was printed, and to urge them to support the board.