Accused rogue trader Kweku Adoboli said on Monday he had lied to his employer UBS about how his trading losses of $2.3 billion were incurred because he had wanted to protect his colleagues from the fallout.

The former fund trader told a London court he had "naively" taken sole responsibility for the losses in an email because he thought the worst that could happen would be that he would lose his job, and he did not wish to wreck other people's careers.

Adoboli, 32, was arrested at UBS offices in the early hours of September 15, 2011, about 13 hours after telling the Swiss bank in an email that he had been booking fictitious trades to mask "off-book" positions.

He has pleaded not guilty to four counts of false accounting and two of fraud by abuse of position.

Describing his state of mind at the time when he sent the email to the bank, he said: "I don't want anyone to take the heat for the losses."

During his long-running trial, Adoboli has argued that his methods were known to others within UBS, that everything he did was for the benefit of the bank, and that UBS management tolerated rule-bending as long as it was profitable.

Prosecutor Sasha Wass asked him why, if that was the case, he had not said so in his "bombshell email", or during lengthy meetings with UBS managers and lawyers later that day.

"I was trying to protect everybody, not just the desk, not just the senior guys, but also the back office," Adoboli said.

He said that if he had known he would be accused of committing a crime, he would have told the truth about the involvement of others earlier than he did.

"I didn't think this amounted to a crime, so there would be no need to call lawyers or the police," he said.

"I expected that I would explain the trades to them and then I could go home."

"A VERY DEVIOUS LIAR"

Instead, Adoboli was arrested and taken to a police station, and then to prison, where he remained for nine months until he was granted bail in June.

Wass took Adoboli through the "bombshell email" and a detailed account of the subsequent meetings, written by a lawyer representing UBS. He said that many of the details were untrue and that had lied to protect others.

Wass put it to him that part-way through one of the meetings he had in fact started to implicate his three colleagues on the Exchange Traded Funds desk.

He accepted that, arguing that by that time he was under considerable pressure as he had been quizzed for six hours, and he had come to realize that he could not fully protect his colleagues.

But he said that, even then, he had lied to conceal the full extent of their knowledge.

"I had to give them a get-out," he said. "By saying what I did say, I was able to give them this level of deniability."

Wass put it to him that he had changed his story as he went along in order to better protect himself, and that he was still doing so at his trial.

"You are a very devious liar," she told Adoboli. "You have carefully weighed up what could be denied, what could be proved ... That is how you have conducted your defense, carefully crafted lies."

The trial continues.

(Editing by Kevin Liffey)