An end effector for an electrosurgical instrument comprises a pair of opposing jaws 2, 3, each provided with a sealing electrode 7, 9, and at least one of which comprises first 5 and second 6 portions that are independently moveable one with respect to the other. A biasing mechanism 14 is also provided. The biasing mechanism 14 being such that the jaws 2, 3 are capable of generating a first closure force when the jaws 2, 3 are open beyond a predetermined set separation, and a second closure force when the jaws 2, 3 are closed beyond the predetermined set separation the first closure force being greater than the second. This ensures that the instrument whilst capable of applying a relatively high force when sealing a vessel of relatively large diameter (16, Fig 2) - applies a reduced force when grasping and sealing vessels having a relatively small diameter (18, Fig 3). The predetermined separation is preferably established through the provision of a stop member 15. In a preferred embodiment the biasing mechanism 14 comprises a spring located between the two independently moveable portions 5, 6, of one of the jaws 2.
