Yes, HM Revenue and Customs (HMRC) has taken measures to ensure the online service is secure. HMRC also constantly monitor the services for any potential security breaches. Security measures that have been implemented are:
Authentication: Access by User ID/password (or Digital Certificate).
Secure connection and encryption: All information that you send and receive is transmitted through a 128-bit Secure Socket Layer connection (SSL). SSL creates a secure link between your browser and HMRC's server. You will always know when you are using a secure connection because a padlock icon is displayed on the status bar of your browser. SSL also encrypts data and guarantees that it is not altered between your computer and HMRC's server.
In addition, HMRC is a responsible data user and is subject to the provisions of the Data Protection Act