Of the 2.4m phone calls received by HMRC in August, 1.7m of the these wanted to speak to an adviser.

However, less than three quarters of them managed to get through to one, leaving over 426,000 taxpayers in the lurch. For the year to date this figure decreased to 62.3% of the 15.1m phone calls received, meaning 3.8m taxpayers have had their questions go unanswered over the year.

After criticism of service levels, HMRC has managed to improve the waiting time for callers to an average of 17 minutes and 24 seconds, down from 28:34 minutes in April.

The percentage of callers waiting more than 10 minutes had improved to 62%, from 75% of long waiters in July.