An alleged rogue trader accused of Britain's biggest fraud wept as he told a jury he was trying to cover millions of pounds worth of losses incurred at the bank he called his "family".

Kweku Adoboli, 32, is accused of gambling away £1.4 billion while working for Swiss bank UBS during the global financial crisis.

At one point he was at risk of causing the bank losses of $12bn (£7.5bn), jurors at Southwark Crown Court were told.

Adoboli, a former public schoolboy, of Whitechapel, east London, broke down in tears as he gave evidence for the first time in his trial, in which he claimed his off-book trades were to cover $40m annual losses of his portfolio of companies from 2008.

The court heard that by 2007 Adoboli, then aged 27, and a more senior trader, John Hughes, 25, were in charge of a portfolio of companies with assets of $50bn.

"Our book was massive. A tiny mistake led to huge losses. We were these two kids trying to make it work," he said.

Adoboli, wearing a dark suit and red tie, denied he was a "gambler" and said his knowledge of UBS's systems did not result in "fraudulent behaviour".

He said: "It's hard to find the words to describe the relationship I had with UBS as an organisation. It isn't about a bank. It was about what I thought was my family, considering how much [I] neglected my real friends and family.

"Every single bit of effort I put into that organisation was for the benefit of the bank, the people around me and the book I worked on.

"If I was not so proud to work for UBS, I would never put so much effort trying to convince them we could achieve something at this bank."

He added: "To find yourself in Wandsworth Prison for nine months because all you did was work so hard for this bank ...", before stopping as he broke down in tears.

Ghanaian-born Adoboli is facing two counts of fraud and four counts of false accounting between October 2008 and last September, allegedly gambling away the money on high-risk illegal trades aimed at boosting his annual bonuses and job prospects.