After the death of his father, Cnut, in 1035, his legitimate son Harthacnut was the most obvious successor. However, he was occupied with the responsibility of ruling his Danish kingdom, so his half-brother Harold "Harefoot" was offered, initially as regent, in his place. His crown was never secure, threatened by challenges from the sons of Aethelred "the Unready"; when he died in 1040, Harthacnut was preparing an invasion force.
Editors. "Harold becomes King". The Literary Encyclopedia. First published 31 July 2012
[http://www.litencyc.com/php/stopics.php?rec=true&UID=1947, accessed 13 February 2016.]