Brown faced journalists at a news conference a day after Labour's general secretary Peter Watt stepped down, acknowledging he was aware a key donor was channeling donations through associates to dodge transparency rules.
Businessman David Abrahams said he gave Labour 400,000 pounds (US$825,000; EUR556,000) through the accounts of others.
Brown said this was "completely unacceptable, cannot be justified and this behavior should never happen again in future."
The British Prime Minister cuts a sociopathic figure, isolated, stubborn beyond belief, totally wrong and convinced that the world gravitates around her feet
The Basmanny Court of Moscow arrested Michael Calvey, the founder of Baring Vostok investment fund, on allegations of embezzling 2.5 billion rubles from Vostochny Bank. Calvey will be held in custody until April 13