Tostig had already endeavoured by personal persuasions to induce Sweyn to revive his claim to the crown of England, and to undertake its conquest; but he altogether declined to undertake so dangerous and difficult an enterprise, and Tostig had then turned to Harold of Norway. Whether his interview with him was before he went to Scotland or whether he went thence to Norway is a point on which historians differ. Some deny that any interview took place, but the balance of probability lies strongly in favour of an early interview, at which Harold entered heartily into Tostig's plans, and began at once to make preparations for the enterprise.

It was certain that an invading force from Norway would land in Northumbria, and Harold, although he might not be able to rely greatly upon the assistance of the northern earls as against the Normans, knew that they would do their best to defeat an expedition landing on their own shores, especially when Tostig was a sharer in the invasion. His own thoughts were wholly bent upon repelling the mighty expedition gathering in Normandy, and for this purpose, by immense efforts, he collected the greatest army and fleet that had ever been got together in England. An incessant watch was kept up along the coast where the Normans might be expected to land, while the fleet cruised for months between the Thames and the Isle of Wight prepared to give battle to the invaders.