European Union (Withdrawal) Bill

My Lords, a point was made by several noble Lords as to a delay in the operation of Article 50. If I remember rightly, under Clause 2, there is a period of two years. Can that be invoked unilaterally or does it need the consent of all 27 other members? I would be most grateful if the Minister could reply. If not, I have no doubt that the noble Lord, Lord Kerr, will correct us.