The charity’s warning comes as a National Audit Office (NAO) report found that lengthy waits to HMRC’s taxes helpline had taken the cost incurred by customers to £97m in 2015/16, with some people waiting over an hour.
Citizens Advice, which helped with 250,000 queries in the last 12 months that could require people to contact HMRC, said lengthy phone waits put people at risk of getting into debt, either because of the cost of the call or missing tax deadlines.
Chief executive Gillian Guy said: “Waiting on the phone to HMRC has put some people at risk of debt.
“Long waiting times not only cause frustration and increase the cost of the call, but can also mean people miss important deadlines.
“For example if you don’t return your tax form on time you face a fine - which for some households can be an additional cost they can’t afford to pay.”
In its report, the NAO said HMRC had cut its number of call handlers before other changes were properly in place, leading to the collapse of service experienced by personal taxpayers in 2014/15.
HMRC's performance deteriorated even further in the first seven months of 2015/16, the NAO said. Average call waiting times tripled compared to 2014-15 levels, hitting 47 minutes for some self-assessment callers during the deadline week for paper tax returns in October 2015.
HMRC recruited extra staff to cope with the problem and performance had recovered, with waiting times improving to an average of five minutes during the week for online returns in January.
The NAO said call waiting times increased significantly between 2012-13 and 2015-16, with some customers having to wait more than an hour for an advisor at busy times.
The increase in waiting times meant the cost to customers – either through the cost of their calls or the economic cost of the time they spent on the phone – rose by more than half.
The NAO estimated that the overall cost incurred by customers who called the taxes helpline increased from £63m in 2012/13 to £97m in 2015/16.
Customers paid £2m less in call costs because HMRC moved to local-rate ‘03’ telephone numbers, but the jump in the economic cost of time spent waiting for an answer or speaking to an advisor counteracted that saving, the NAO found.
Amyas Morse, head of the National Audit Office, said HMRC had got its timing “badly wrong in 2014” by letting call handling staff go before a new digitally-led focus was working properly, leading to a “collapse in service quality”.
He said: “HMRC needs to move forward carefully and get their strategy back on track while maintaining, and hopefully improving, service standards."
Ms Guy added: “It will be a relief to callers that average waiting times to get through to HMRC have reduced since last year.
“It is really important that HMRC continues to use the data they have on when people are calling to better manage their resources and meet demand, especially ahead of the deadline for tax credit renewals at the end of July.”