Define the estoppel argument.

The estoppel argument: A MS which has not adopted the implementing measures required by the directive in the prescribed periods may not rely, as against individuals, on its own failure to perform the obligations which the directive entails. In other words: A MS that fails to implement its EU obligations is 'estopped' from invoking that failure as a defence, and individuals are consequently and collaterally entitled to rely on the directive as against the State