On 3 December 1917 at Masnieres, France, during a very heavy bombardment Captain Lascelles, although wounded, continued to encourage his men and organize the defence until the attack was driven off. Shortly afterwards the enemy attacked again and captured the trench, taking several prisoners. Captain Lascelles at once jumped onto the parapet and followed by his 12 remaining men rushed across under very heavy machine-gun fire and drove over 60 of the enemy back. Later the enemy attacked again and captured the trench and Captain Lascelles, who later managed to escape in spite of having received two further wounds.

He was killed in action, Fontaine-au-Bois, France, on 7 November 1918.