From September 2017, HMRC will make it a requirement for all businesses using their online tax accounts to register for 2 Step Verification (2SV). HMRC say this change will mean greater security for customers and a safer experience when using online services. 

The 2SV process involves giving HMRC a mobile number when a tax account is created, to which it sends a six digit verification code whenever anyone attempts to log into that account. 

HMRC began introducing 2SV in December 2015 and since then more than 11 million individuals and businesses have successfully set up 2SV to protect their online accounts.

If you encounter issues using 2SV with HMRC, please contact their Online Services Helpdesk: 

https://www.gov.uk/government/organisations/hm-revenue-customs/contact/online-services-helpdesk