Harbinger Capital, the activist American hedge fund investor, admitted making a tentative bid approach to Inmarsat yesterday, sending shares in the £2.4 billion satellite operator soaring.

The London-listed company, which specialises in satellite technology involving mobile phones, jumped as much as 10 per cent after Harbinger's admission ended months of speculation. Shares closed up 30p, 6 per cent higher, adding almost £140 million to Inmarsat's market value.

Both companies described the approach as being in its early stages and they offered no certainty that a formal bid would emerge soon. Harbinger said that it had yet to decide whether to press ahead with a firm bid for Inmarsat, noting that obtaining regulatory approval would be a lengthy process.

If it wanted to pursue a takeover, Harbinger would have to gain clearance from the Federal Communications Commission, the American industry watchdog that oversees the sector in a similar way to Ofcom in Britain. This process could take up to 18 months. Banking sources suggested that Inmarsat might be able to apply to the UK Takeover Panel for an exemption, which would mean that it would not remain in an offer period for the entire time.

Harbinger, which has accumulated a 29.9 per cent stake in Inmarsat, is likely to approach the American regulator in the coming days now that its interest has been made public.

Sources close to the fund played down the prospect of any formal bid for at least 12 months. One City source close to the deal said: “Inmarsat can either choose to help us out working through the regulatory process, or it can choose not to. What we hope it will not do is brief against the idea of our involvement.”

Harbinger is run by Philip Falcone, a former trader at Barclays Capital, and operates funds under management of $26.5 billion (£13.4 billion) as of the end of June. It was forced to admit that it was lining up a potential offer after it emerged that Harbinger had hired Merrill Lynch, the American investment bank, to review the possibility of a bid. Merrill has a longstanding relationship with Harbinger in the United States, but this is the first time that the bank has advised it on a European deal.

Sources close to the fund said that Harbinger wanted “to take control of the company, just like a traditional strategic buyer”.

Aside from its Inmarsat shares, Harbinger has become known for its big position in Tate & Lyle, the sugar producer, of which it holds 19.25 per cent. It also emerged last month with a 3.29 per cent short position in HBOS, the bank. The fund was forced to disclose its big bet under new rules imposed by the Financial Services Authority, the City's regulator.

Harbinger is one of a handful of hedge funds to have piled into the satellite group's shares in recent months. Funds run by Lansdowne Partners and Lehman Brothers have also been big buyers in anticipation of consolidation in the industry.

Harbinger, which owns stakes in SkyTerra and Terrestar, both rival operators, is also attracted by the prospective role that Inmarsat might play in the development of a satellite-based mobile phone network in the US.

Inmarsat and Harbinger declined to comment further.

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