HM Revenue and Customs (HMRC) has closed most of its phone lines due to a fault as many people seek advice ahead of filing their tax returns. The tax authority apologised for the outage and urged people to use their online services. It is not known when the lines will be working again, but HMRC said it would "reopen them as soon as possible". More than 12 million people are required to complete self-assessment tax returns before 31 January. That number is made up of people who are self-employed and those with more than one source of income. If people do not file their tax return, either online or via post, then they face being hit with a fine, which is usually £100.