Goldman Sachs and JP Morgan are among the big names pitching for a key advisory role with Cairn Energy, the Edinburgh-based group that explores for oil from the North Sea to Kathmandu.
Bank of America Merrill Lynch's role as one of Cairn's two brokers, which act as intermediaries between companies and shareholders, is under threat.
Although Merrill is re-pitching, the possibility that the bank could be removed is the latest fallout from losing its leading oil & gas broker, Andrew Osborne, last year.
Mr Osborne was accused of market abuse by the Financial Services Authority and left Merrill. He has since accepted a £350,000 fine but has retained his banking licence.
Soon after his departure Merrill lost FTSE 100 client Tullow Oil. The bank has suffered other high-profile departures, such as former co-head of broking Mark Astaire, who joined Barclays Capital.
An industry source said that leading banks view Cairn, which made a $4.6bn (£2.9bn) post-tax profit in 2011, as "a good opportunity in the broking space". Cairn hit the headlines last year when it sold a large chunk of its Indian operations to Vedanta Resources for $8.7bn.
Cairn has since dropped out of the FTSE 100, though shareholders had been rewarded with a $4.5bn cash hand-out after the Indian sale. The group's other broker, Morgan Stanley, is thought to be secure in its role, while insiders said Merrill still had a strong enough oil & gas team to have a chance of holding on to its brokerage.
Mr Osborne has since joined the private equity-backed North Sea oil company Chrysaor as its finance director.