Some sources claim that Harold took part in William's Breton campaign of 1064 and that Harold swore to uphold William's claim to the English throne at the end of the campaign, but no English source reports this trip, and it is unclear if it actually occurred.
Nogle kilder påstår, at Harold deltog i Vilhelms felttog i Bretagne i 1064, og at Harold svor at opretholde Vilhelms krav om den engelske trone mod slutningen af felttogt, men der er ingen engelske kilder, der beskriver denne rejse, og det er uklart om den rent faktisk fandt sted.