Accused UBS "rogue trader" Kweku Adoboli said on Friday he did not believe his behaviour had been fraudulent and it was unfair to suggest that his previous jobs in the UBS back office had given him the knowledge and skills to commit fraud.

Adoboli, 32, who worked for the Swiss bank as a trader in London, was arrested on Sept. 15, 2011 and blamed for losses of $2.3 billion. He denies four counts of false accounting and two of fraud.

Answering questions from his defence counsel in a packed London courtroom, Adoboli said he had been asked to bring his "ability to understand the interaction of systems" to the Exchange Traded Funds (ETFs) desk when he joined as a junior trader in 2006.

"It is not fraudulent, it is finding a way to do your job," Adoboli said.

After crying within seconds of entering the witness box, he shed tears again when talking about Mike Foster, who was in charge of the ETFs desk when Adoboli joined and who has been supporting him throughout his trial. Foster no longer works for UBS.

"He wanted to make sure we could build something we could all be proud of," Adoboli said, his voice choked with emotion, after a long pause during which tears welled up in his eyes and he was unable to speak.

The judge told Adoboli he should not be embarrassed by these moments of emotion, that it was clear he was becoming emotional when talking about people he cared about such as his father or Foster, and that it was useful for the jury to get a sense of his personality.