Mohammed Ahmed, a bank manager at Barclays, is accused of helping fraudsters steal more than £820,000 from a pensioner's nest egg.
Mohammed Ahmed, 26, first made unauthorised changes to the savings account set up by 75-year-old Brian Mahoney, it is claimed.
He then transferred all but £80,000 out of the retirement fund, jurors were told.
Mr Mahoney, from Cheshunt, Herts, only found out when he rang the bank two months later to check how much interest he was receiving, the Daily Mail reports.
He told the Old Bailey he set up the Nest-egg savings account in October 2008 with a payment of £1m because of the banking crisis.
"Everyone was moving money, and I was looking for the best terms obviously.
"Until I went to phone up to get the balance I didn't realise."
He did not touch the account other than to transfer £100,000 to his daughter, the court heard.
But Ahmed made repeated and unnecessary checks of the account in the period leading up to February 2009, it is claimed.
Then on February 20, 2009, he changed it to a joint account between Mr Mahoney and another man, Philip Mubaru, jurors were told.
Records suggested that both men were present and Mr Mubaru had presented his passport and driving licence.
But prosecutor John Carmichael told the court: ‘The Crown say that is not what happened and that was Mr Ahmed accessing the account and manipulating the account.
Ahmed was arrested after Mr Mahoney alerted the bank in May 2009 and denied any fraud.
He denies two counts of fraud and the trial continues.
Data quality is key
Granted leave to appeal the judgement