Aviva has promised to compensate clients who may be unable to file their tax returns on time because of replatforming problems.
Advisers who use the platform typically receive a tax statement, which contains a breakdown of the income earned and assets held of their client, in June and pass this onto the client, who then uses it to prepare a tax return.
But Aviva only managed to send these tax statements out this week - five months later than normal.
The self assessment tax deadline is 31 January.
Minesh Patel, an adviser at EA Financial Solutions in London, said he uses the Aviva platform for some of the biggest clients of his firm.
He said the reduced amount of time his clients will have to complete their tax return could impact on their ability to engage in proper tax planning.
Mr Patel said: "I have angry clients ringing me up, complaining. I was promised by Aviva at the start of September that the tax vouchers would be sent within a few days, the same at the start of October.
"Then I was promised I would have them by October 31 and it hasn't happened. This is completely unacceptable."
A spokesman for Aviva said: "Tax vouchers are usually sent out in June, but they have been delayed this year because of issues arising from replatforming.
"We apologise for this delay and will reimburse customers who submit their tax return by paper and incur a late submission fee because of this issue.
"This has affected customers who receive dividends or other income payments with their General Investment Accounts, if they are required to submit tax returns and do this through the paper tax return channel."
HM Revenue & Custom's fine for late filing of a personal self-assessment tax return is £100 if the return is made within the first three months, though the penalty is higher if the tax bill is then paid late.
Mr Patel said in the past Aviva was his platform of choice for new clients, with it receiving a greater proportion of his new clients than any other platform.
However he said the litany of woes he has experienced as a result of Aviva's replatforming means it is no longer the fund supermarket he recommends to the bulk of his clients.
The platform was unavailable for six days beginning on January 17 for replatforming.
But when it came back online many advisers found issues with the new client reporting function and technical issues which affected payments for people in drawdown.
Advisers also reported they were not getting their payments through the platform.
But Mr Patel said the regulatory and time burden of switching clients away from a platform was prohibitive so he feels forced to stick with Aviva for the moment.
Aviva has a history for failing to hit self-imposed targets to address issues raised by the replatforming, with the company having stated in March that it would have restored the client reporting function within a week.
Aviva missed this deadline.
The company has refunded clients for previous errors made on during the replatforming, though one adviser, Stephen McDine, an adviser at Newcastle-based firm Positive Wealth Management, expressed dis-satisfaction at the level of compensation he received from Aviva.
Last month Adrian Durham, chief executive of platform technology provider FNZ, said his company accepted responsibility for the problems with Aviva's replatforming earlier this year.
He said his company had learnt from the debacle and would make sure the issues did not reoccur during other migrations FNZ is involved in, such as the looming Quilter replatforming.
Mr Durham said: "Aviva was entirely to do with the transition from the previous system and service provider to FNZ.
"Transitions are extremely complex, which involved moving multiple hundreds of millions of rows of data around.
"Clearly Aviva was less successful than it could have been and we accept responsibility for that and have poured a large amount of resources into addressing that issue.
"We have had a number of other large scale migrations which have not made it to the papers. It costs us a huge amount of money when we get things in any way wrong."
david.thorpe@ft.com
