Life insurance and increase in premiums

My parents have had an indexed linked policy since 1988.
The premium rose every year in line with inflation

Recently the policy has been reviewed and they have been told they will need to double their premiums to receive the same payout on death.

The letter advises that they are getting older etc...and are now higher risk

I assume the insurance company are within their rights to do this?

The terms & conditions of the original policy should clarify.

Do they need this cover? Do they have dependents?

Sounds like another unit linked whole of life policy. They should really get professional advice.