A method of reinforcing a metal panel or pipe of an existing structure comprising the steps of attaching a reinforcing metal layer to said metal panel or pipe in spaced apart relation to thereby form at least one cavity between inner surfaces of said metal panel and said reinforcing metal layer, injecting an intermediate layer comprised of an uncured plastics material into said at least one cavity, and curing said plastics material so that it adheres to said inner surfaces of said metal panel or pipe and said reinforcing metal layer.