Many viewers are reporting on Twitter that they are able to watch ITV in high definition without any problems, and that ITV +1 is working.
More top news
See how your area fares.
The world's first ever 'hole in the wall' was installed in Enfield, north London, on June 27 1967.
America says it has "identified preparations" for an attack similar to that which killed at least 86, including 30 children, in April.