The taxman has confirmed that it will appeal a ruling over discount tax being applied to "loyalty bonuses" given to Hargreaves Lansdown investors.
HMRC were defeated in a ruling in March over its right to tax the bonuses, which were introduced 15 years ago and are a discount provided to investors.
The dispute began in 2013 when the HMRC changed its stance on whether "loyalty bonuses" should be hit by a 20 per cent tax.
It now says that rebates of annual charges (such as loyalty bonuses) paid on funds held outside ISAs or SIPPs should be taxed as income and paid net of basic rate tax.
But Hargreaves Lansdown has appealed this decision, stating that the HMRC's position was an "unwarranted attack on private investors."
While the ruling is ongoing, the tax is being applied to the bonuses, but another decision in Hargreaves Lansdown's favour could see £15m returned to investors.
"Following the decision by the first tier tax tribunal in our favour, we see no reason why we would not be successful at appeal," Hargreaves Lansdown chief exec Chris Hill said.
"The process is likely to complete in the first half of 2019 and a successful outcome will see millions returns to clients, as well as a simplification of their tax affairs.
"The 'discount tax' has always been an unnecessary and unwarranted attack on private investors."
A spokesperson for HMRC said it was "disappointed" by the judgement and confirmed it would be appealing.
"The current rules will continue to apply until we are satisfied that the litigation process is complete.
"HMRC have been granted permission to appeal by 11 June."