The Swiss bank has finished its discussions with Finma, the Swiss regulator, and is finalising a settlement with Britain's Financial Services Authority (FSA), the source said. The two regulators then plan to make a joint announcement.
The FSA could fine UBS between 20 million and 50 million pounds, the Financial Times said earlier on Thursday, adding that a deal could come next week.
The two regulators are jointly investigating the bank's control failings that led to Kweku Adoboli, a trader on UBS's "Delta One" desk, losing $2.3 billion for the bank.
That investigation is continuing after being put on hold during Adoboli's criminal trial. He was convicted of fraud and jailed for seven years on Tuesday.
UBS is keen to put the rogue trading scandal behind it, which could speed up any settlement with regulators.
Finma does not have the power to impose fines. It is expected to impose new supervisory conditions on UBS.
UBS, the FSA and Finma all declined to comment.
Any fine on UBS could be the second-biggest ever levied by the FSA, after its 59.5 million pound penalty on Britain's Barclays Plc
UBS is also one of about a dozen banks being investigated for manipulating Libor, which could lead to further hefty fines being meted out. It is also in the middle of a big overhaul of its investment bank arm as it tries to cut costs and boost profits.