Question

The Prime Minister of UK is the keystone of the Cabinet

Answer 1

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Prime Minister is the most important functionary in a Parliamentary form of government. Right to say Pt. Nehru called the Prime Minister as the "Linchpin of the government":

a.) He is the head of the government. All the members of  the council are appointed by the President on the recommendation of the Prime Minister. It is the Prime Minister who allocates portfolios to the minister.

b.) He is the head of the cabinet, presides over its meetings determines its business to be transacted, takes all important policy decisions.

c.) He acts as link between the Cabinet and the President. He communicates all major decisions of the Cabinet to the President.

d.) The Prime Minister is the keystone of the Cabinet arch. As Laski said, "Prime Minister is central to the formation of Council of Ministers, central to its life and Central to its death..." Prime Minister can remove any Minister, dismiss him , reshuffle him. 

Most Importantly, if the Prime Minister resigns or dies the entire cabinet falls like a pack of cards.

e.) Prime Minister is the leader of the Parliament. He is chief spokesman of the government in the House and acts as a link between the President and the Parliament.