Brian Walters, who was the Newcastle bank's senior commercial lending manager for Yorkshire, also attacked the tripartite system of regulation which failed to prevent the first run on a UK bank for 141 years.

Mr Walters, whose Northern Rock office loaned up to £100m in a good year, said the Rock board's apparent policy of trying to slow the massive growth in its mortgage lending never reached its own managers.

It was the bank's unsustainable expansion of its lending scheme, when it did not have enough held in deposits and relied on inter-bank loans, that forced it to seek emergency funding from the Bank of England, the lender of last resort.

Mr Walters, of Harrogate, told the Yorkshire Post: "The management was growing the business at the rate of knots. Had the bank not grown at such a rate the crisis would not have happened.

"Adam Applegarth, then the chief executive, said he took the decision to slow down in April 2007 but nobody I spoke to was aware of that. If he arrived at that view then he did not communicate that to staff.

"They said to the outside world 'we are OK' but they were fast running out of money."

Now the 54-year-old, who left the Rock earlier this year to join another building society, has written a book, The Fall of Northern Rock: An insider's story of Britain's biggest banking disaster.

In it he describes the time he spent at the Rock, which he joined in 2005, its growth, the run on the bank, the attempted takeovers by Sir Richard Branson's Virgin Group and by the Rock's own board, and how it was nationalised, or taken into "a temporary period of public ownership", in February this year.

While he said that several factors contributed to the failure of the bank, and that "almost everything that could have gone wrong did so", he concludes: "Fundamentally, it is the role of management to ensure that a bank remains solvent and able to meet its commitments. In that regard the board of directors failed in its duty."

Mr Walters said the board knew that "something needed to be done" by spring last year but the bank's commercial loan book was still growing at 20 per cent a year.

The Rock had also introduced the now notorious – and since scrapped – "Together" home loans, which offered customers a sum of up to 125 per cent of the value of their property.

But, as well as the unfettered growth of the bank, Mr Walters also said the regulatory system of the Bank of England, the Financial Services Authority (FSA) and the Treasury, introduced by Gordon Brown when he was Chancellor, failed to provide the necessary checks and balances.

"The Bank, the FSA and the Treasury had never had to deal with this before and they got it wrong."

He said it was unclear who would take a lead when such a crisis developed and that the tripartite system had failed because its three arms could not agree completely on how to handle the situation.

But although Mr Walters describes the credit crunch as a "serious global crisis", he said the failure of the Rock, and the Bank's unprecedented bailing out of it, may prevent another run on a UK bank. There is "no question" of a run at the embattled Bradford & Bingley, he added.