A BARCLAYS security chief accused of trying to blackmail the bank for £25m has told an Old Bailey jury he was pushed over the edge by a £4 overdraft fine.

Graham Browne, who led the cryptography team, admitted writing four letters to Barclays' chief executive asking for the payout.

But he was merely trying to demonstrate how simple it is to defraud the system, he claimed.

He said his ultimate aim was to prevent the bank scrapping one of his team's projects relating to the security of customer cash cards.

Browne later told the jury why he became angry with the bank: 'It was very, very trivial. The bank tried to charge me £4.15 for an overdraft they had created in the first place.'

Browne, 57, of, Knutsford Road, Cranage, Holmes Chapel, denies one charge of blackmail dating between March 8 and September 27 last year.

He told the court he became increasingly concerned over the bank's security during his 13 years there.

He said his job to encript PIN numbers on customers' cash and debit cards became worsened by bad management.

'They felt it was better to buy something from IBM, which must be good, than have to develop the idea in-house by their own staff,' he said.

He never had enough staff to 'do a good job' and would have to beg senior management for resources, he told jurors.

The divorced father told how his stepdad died in 1998 and, during the resolution of his estate, his Barclays account was plunged more than £8,000 overdrawn in error.

'The bank recognised the error, apologised and sent two bottles of wine, one red, one white,' he said. 'Shortly after that, they sent me the bill for the wine.'

The bank then charged him £4.15 in overdraft charges despite admitting their error, he said.

Browne said this tipped him over the edge but admitted he felt 'highly unstable and  depressed'.

He was refused voluntary redundancy and so resigned by e-mail in January last year, he said.

Weeks later he wrote the first of four letters to the bank's chief executive, the court heard.

He said: 'My aim was for Barclays to start an investigation. It was not a device to get money for myself and I was not contemplating releasing the information. I wouldn't even know how to.'

The prosecution claims he became 'disillusioned, bitter and angry' because Barclays did not acknowledge his capabilities.

The trial continues.