People calling the taxman could find themselves waiting around 38 minutes to get through - or more than an hour if they call in the evening, an investigation by Which? has found.
Which? made 100 calls to HM Revenue and Customs' self-assessment and general enquiries helplines between September and early October to find out the time it takes to get through to an adviser, ahead of the self-assessment tax return deadline on January 31.
It found that on average, it took researchers 38 minutes to speak to someone - which is more than double the time it waited last year, when similar research was carried out. Nearly one in five (18) of the consumer group's calls was not answered within an hour.
Not one call made by the consumer group was answered within 15 minutes.
Which? also found that the later in the day it called, the longer researchers had to wait.
Before 2pm, the average wait was 28 minutes, between 2pm and 6pm it was 49 minutes and after 6pm the average wait was 61 minutes. The longest Which? had to wait was one hour and 16 minutes.
But HMRC said the Which? figures do not paint an accurate picture of its current call handling performance, which has been significantly improved, with people now queuing for around six minutes typically.
When Which? carried out the same investigation in 2014, the average wait time was 18 minutes, with nearly a third (29) of calls cut off before it got through to someone.
In the last year, HMRC has changed its phone system to allow people to hold for longer rather than being cut off at busy times, so while wait times went up this year, only seven calls were cut off.
A report in November from the Public Accounts Committee (PAC) found that the level of customer service provided by HMRC had become so poor that it could be considered a ''genuine threat to tax collection''.
In October, Which? surveyed 245 of its members who had contacted HMRC by phone in the last 12 months. More than half (58%) of those surveyed would only want to wait for up to five minutes.
Which? executive director Richard Lloyd said: "Our findings show how difficult it is to get through to HMRC's helplines with people facing lengthy waits.
"With many people soon to be seeking advice on their self-assessment tax return, HMRC must continue to work hard to improve customer service and reduce call-waiting times."
Which? has a free-to-use tax calculator to help people to check if they are overpaying tax online rather than being kept waiting on hold. The tax calculator can be found at money.which.co.uk/tax-calculator.
An HMRC spokesman said: "These figures are out of date and our service has significantly improved in recent months, with average queue times now at around six minutes.
"Our service levels were not good enough at busy periods earlier this year, and we have apologised for the inconvenience caused to our customers.
"We took major steps to improve, including the recruitment of 3,000 new staff into customer service roles, and we expect service levels to continue to rise. As last year, we will be putting even more people on our phone lines over the next few weeks to support customers completing their self assessment tax return by 31 January.
"But while it's right we are there on the phone when people need us, in the future there will be less need to call as people will be able to do much more online through digital accounts, just as they do with online banking."