(Reuters) - British bank Shawbrook Group Plc (>> Shawbrook Group PLC) said it expected to book an additional impairment charge in the second quarter due to some irregularities in its asset finance business and that Chief Financial Officer Tom Wood had resigned.

The charge of about 9 million pounds ($12 million) relates to a number of loans being underwritten in its asset finance business that did not match its lending criteria, the company said.

Shares in Shawbrook fell as much as 28 percent to a record low of 117.3 pence on Tuesday, making them top losers on the FTSE midcap index <.FTMC> in a positive market.

The announcement follows two consecutive sessions of sharp drop in UK banks, spurred by massive sell-offs amid uncertainties after Britain's decision to leave the European Union.

Shawbrook has lost more than half its value since Thursday's close, in tandem with falls at other new banking entrants, or so-called "challenger banks", such as Virgin Money Holdings Plc (>> Virgin Money Holdings (UK) PLC), Aldermore Group Plc (>> Aldermore Group PLC) and OneSavings Bank Plc (>> OneSavings Bank PLC).

Shawbrook, which was created in 2011, said that although the charge would have an impact on full-year pretax profit, its performance elsewhere remained in line with its own expectations and second-quarter loan originations were up about 35 percent from last year.

It had reported 2015 underlying pretax profit of 80.1 million pounds.

Citi said its UK economists were predicting a cut to the country's GDP following the leave vote - an outcome that could impact loan growth, especially among small- and medium-sized businesses.

Shawbrook, with a 75 percent exposure to SME lending, is most exposed to tougher environment that would result in slower volume growth, a deterioration in asset quality and lower margins, Citi analysts wrote and downgraded Britain's challenger banks to "sell".

Separately, Shawbrook said CFO Wood would leave on June 30. This means the bank would see a change of chairman, CEO and CFO since going public last year.

Wood wanted to spend more time at home, Shawbrook quoted him as saying.

Shore Capital analyst Gary Greenwood said the company had assured him that Wood's departure was unrelated to the breach, which had "severely damaged" confidence and could delay plans to start dividends.

Shawbrook said it was confident that such a breach would not recur as it had upgraded its risk management systems and controls.

The company named Dylan Minto, director of strategy, as interim CFO.

($1 = 0.7528 pounds)

(Reporting by Esha Vaish and Noor Zainab Hussain in Bengaluru; Editing by Sunil Nair and Gopakumar Warrier)