NBNK, which was run by former Barclaycard chief Gary Hoffman, competed against the Co-op in earlier rounds, alongside Hugh Osmond’s Sun Capital. However when the Co-op deal ended, Lloyds went ahead with plans to float the business, culminating in last week’s intention to float announcement.

Crystal Amber, which is chaired by William Collins, the former Barings private client banker, is believed to be preparing to make a public statement regarding its intentions with NBNK as early as this week.

Since failing to buy the TSB branches or find another banking business to acquire, NBNK’s operations have been scaled back and costs reduced to a minimum.

However it has retained its listing, and at the end of December had £20.2m of cash on its balance sheet, according to its latest results.

Crystal Amber’s presence on the NBNK share register means the cash shell — which has a market capitalisation of £15.3m based on its Friday night share price of 28.5p — has two activist investors on its register.

Wilbur Ross, the US investor, owns a 29.9pc stake and has been vocal in stating he would like to use the vehicle to buy up UK bank assets.

It is thought Crystal Amber’s desire to sue Lloyds may well find favour with former chairman, Lord Levene of Portsoken, who has been outspoken about the sales process and the way NBNK was treated.

He told MPs that Lloyds was “swayed by political considerations” when it came to choosing the Co-op.

A spokesman for Crystal Amber declined to comment.