CAIRO, Sept 12 (Reuters) - A proposal by ultraconservative Salafis to give Egypt's main Islamic institution the final say on whether the law of the land adheres to Islamic laws threatens to bring the already painfully slow process of drafting the new constitution to a grinding halt.

The proposal would give the revered Al-Azhar power similar to a supreme court by making it the arbiter of whether a law conforms with the principles of sharia, already cited in the constitution of ousted leader Hosni Mubarak as Egypt's "main source" of legislation.