Campaigners for around one million former Bradford & Bingley shareholders have pledged to step up efforts to find out why the bank was part-nationalised and part-sold off at the height of the UK banking crisis.

The Bradford & Bingley Shareholders’ Action Group will use the run-up to the General Election to press ministers to explain why B&B became the only bank to be split up.

Over a dramatic weekend in September, 2008, Spanish bank Santander acquired the £20 billion B&B savings book and its 197 branches for £612 million, and the Treasury took control of the mortgage business, which ceased lending.

The Bradford & Bingley brand name is set to disappear from the high street this month as all the Spanish bank’s UK subsidiaries adopt the Santander name.

Since the B&B rescue, campaigners have called on ministers to explain why it was done in this way, and also called for a Serious Fraud Office enquiry into the move..

Shareholders’ group chairman David Blundell said: “Bradford & Bingley was the only bank to be split up during the crisis and we have strong suspicions that it was for political rather than financial reasons that this was done.

“Why, after an apparently successful £400m rights issue, did the Treasury take the view that B&B was not a viable business and why was it split up?

“The Government has failed to respond to our repeated requests for full information about the move, and we will use the election period to crank up our efforts to get the full facts.”

Meanwhile, a probe by the City watchdog into Bradford & Bingley and two other failed UK banks has revealed internal breakdowns which allowed risks to go unchecked.

The Financial Services Authority has reportedly been carrying out a review of B&B, Royal Bank of Scotland and HBOS since April.

Although the investigation did not find any illegal activity, communications failures left board members unaware of the risks being run by executives before the crisis struck.

A forthcoming FSA report is also expected to question public statements made by the banks before the series of Government bailouts.

At the last B&B annual general meeting before its rescue, held in Bradford in April, 2008, the-then chairman Rod Kent apologised to shareholders for bad investment decisions.

Finance director Chris Wilford also admitted that less sophisticated IT systems dating from the bank’s building society days had failed to provide the board with up-to-date financial information on which to base decisions.

PricewaterhouseCoopers has been examining RBS on behalf of the FSA, with Ernst & Young looking at HBOS, and BDO Stoy Hayward reviewing Bradford & Bingley, which has faced questions over its buy-to-let loan book and its self-certification mortgages.

The FSA declined to comment on the report.