The radio activity alerted the Admiralty to the threat, and they ordered Support Group 5, comprising escort carrier HMS Biter and four destroyers to join.

At that time SG 5 was supporting HX 237, under attack by boats from Rhein and Drossel, but was then in range of Coastal Command aircraft, and it was decided SC 129 was in more need.

[1] The report by U-504 had brought up about a dozen U-boats, and on the evening of 11 May one of these, U-402, slipped past the escorts and torpedoed two ships.

MacIntyre was, in his own words, "furious" that ships under his group's protection had been sunk;[2] in the previous nine months they had escorted tens of convoys without loss.

MacIntyre organized a vigorous hunt for the U-boat; she was found by HMS Gentian, attacked with depth charges, and so badly damaged she was forced to abandon her patrol and return to base.

[3][4] During 12 May the assembled U-boats made over a dozen separate attempts to penetrate the escort screen, but an aggressive defence by the warships, despite being low on fuel and ammunition, prevented any losses.