In the United Kingdom any income from spread betting and fixed odds trading is regarded as gambling as opposed to trading and therefore is not subject to tax.But in theory spread betting gains aren't really an income and therefore should not be accounted for on a tax return, however my accountant has always been adamant that I should declare all income.
Also, I know of one 'professional' spread better who made the assumption that he didn't need to declare his spread betting winnings but was nevertheless hounded for two years by HMRC. They were eventually slapped down (HMRC) by a judge but he had to pay thousands to a lawyer to defend himself. Be warned, the concept of 'the law' as we know it does not sit well with HM Revenue & Customs. As far as they are concerned, everything is interpretive! This precedent is sufficient for me not to want to go through the same ordeal.
In reality the spread betting companies already pay a gambling tax which is why traders in the UK and Ireland don't pay any additional taxes on winnings - the catch is of course that you can't claim for losses!
