very thin NON combustible material to line bottom of wooden shelf

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I want the name of a product that is fire resistant or at least retardant to attach to the bottom of of my wooden shelf which extends over the back 2 back gas hobs. It must be very thin and effective.

My home was built in the 50″s so the shelf has been in the same place since built.

However lately I have been told that it is outside the regulations, does anyone know of a suitable product, as I will not change the shelf.

Thank you for any input that will aid me in this issue
Josephine