Callers were unable to reach HMRC for a period of four days from the afternoon of Thursday December 1 until Monday December 5. The department also had to shut some online services during this time.

The outage occured as the crucial tax deadline for self-assessment approaches next month. HMRC attributed the system failure to an IT glitch causing web users to experience a “lost connection and slow running service”.

The problems were critical and also affected the New Computerised Transit System (NCTS), which is responsible for facilitating the moving of goods in and out of the country. HMRC advised traders delivering goods to not travel to ports during the outage, The Telegraph reports. 

HMRC chief executive Jim Harra wrote to MPs saying HMRC “decided to ‘shutter’ PAYE Online from users and to close all of our telephony helplines” in a bid “to restore the performance of [the] NCTS and keep freight flowing across the border”.

Services are now operating as normal.