HMRC have so far offered no explanation as to why their online self assessment filing facility has become unavailable to users on deadline day.
The deparment says it is 'working with our IT service provider to give this our urgent attention'.
Users of third-party software have not been affected.
As yet, there has been no suggestion of an extention to the SA deadline.
Rumours are that the cause of the failure is not an overload of the system.
More will follow.