Shawbrook stock posted a double-digit gain on the FTSE 250 on Friday after a London private equity outfit made a firm bid for the bank, which was rejected, and cut the number of shares it is seeking to acquire in order for a deal to be struck.

The fledgling lender, established in 2011, now appears to be facing a siege from buyout shop and largest shareholder Pollen Street Capital. Pollen now wants to buy back Shawbrook just two years after selling a majority stake of it on the London Stock Exchange.

It bid 330 pence plus a scheduled dividend of 2.7 pence, valuing the bank at £842.4 million ($1.05 billion), and pushing the stock up by more than 10% to 334.8 pence.

Shawbrook has rejected the bid, saying that it undervalues the business. Pollen Street Capital had already had a number of informal offers thrown back at it but with Friday's bid, it is now appealing directly to shareholders.

The buyout shop said it will now accept just 50% of the company and one extra share, a simple majority takeover in other words, for a deal to happen whereas before it had said it would seek 75% of the voting rights in the target.

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Pollen was a founding shareholder of Shawbrook and still has a 38.9% stake. It sold nearly half of the bank in a 2015 initial public offering, for 290.0 pence per share, and later offloaded a further 10% of the company at 335.0 pence per share.

The private equity investor said earlier this month that Shawbrook would have better growth prospects if it were taken private as it would have more flexibility over strategy and could retain capital by not paying dividends.

It is possible that Pollen was propelled into action by external and internal events at Shawbrook that have hit its share price over the last nine months.

The lender saw its stock slump by more than 30% in response to the U.K.'s vote to leave the European Union in June only for them to fall by another 30% just days later when it announced a sudden loan impairment in its Scottish mortgage book.

The loan impairment, which followed a breach of internal lending controls, could be why Shawbrook stock often lagged its peer group during recovery that ensued across the banking sector after the referendum. Until the recent offer became public Shawbrook stock remained below its IPO price.