BY PAUL SONNE

LONDON—Kweku Adoboli, the UBS AG trader accused of making unauthorized transactions that cost the Swiss bank as much as $2.3 billion, pleaded not guilty to two counts of fraud and two counts of false accounting in a London court on Monday.

British authorities arrested Mr. Adoboli in connection with allegations of rogue trading in mid-September but the trader entered a plea for the first time only on Monday. The 31-year-old trader had more than once requested additional time to evaluate the evidence and obtain legal advice before deciding on a plea.

In a statement last …