Method of authenticating a client, identity and service providers, authentication and authentication assertion request signals and corresponding computer programs

A method is provided of authenticating a client to access a service provided by a service provider, whereby the service provider queries an identity provider to verify identity of the client and authorize access the service. The method includes: verifying using the identity provider to verify that an identity level corresponding to an earlier authentication of the client is stored with the identity provider, and granting service access authorization to the client, which is performed either (i) directly following the verification step when the identity level required is less than the stored identity level, or (ii) after the following steps when the identity level required is greater than the stored identity level or when no client authentication is available, namely requesting authentication of the client having the required identity level and replacing the stored identity level with the required identity level if the client is authenticated by the identity provider.