A computer-implemented method comprising: receiving, at a client system from a user device, a first access request comprising a first instruction to access a protected resource stored at a resource system; transmitting, from the client system to an authorisation system, a token request for an access token to be used for accessing the protected resource, in response to the first access request; receiving, at the client system, an access token in response to the token request, the access token having a corresponding time to expire indicative of a time at which the access token will not be valid for obtaining the protected resource from the resource system; in response to the token request, receiving and storing at the token storage unit an expiry time indicator corresponding to the received access token's time to expire, wherein the expiry time indicator is indicative of the time at which the corresponding access token will not be valid for obtaining the protected resource from the resource system, wherein storage of the access token at the token storage unit is maintained until a time after the time indicated by the expiry time indicator; transmitting the access token from the client system to the resource system and, in response, receiving the protected resource; storing the access token at a token storage unit of the client system; receiving, at the client system from the user device, a second access request comprising a second instruction to access a protected resource stored at the resource system; and in response to the second access request, transmitting the stored access token to the resource system for receiving the protected resource, characterized by performing, by the client system, token storage maintenance steps, wherein the token storage maintenance steps are executed according to a predetermined schedule that defines a time interval between adjacent executions of the token storage maintenance steps, the token storage maintenance steps comprising: comparing the time indicated by the stored expiry time indicator with a current time; and deleting the access token corresponding with the stored expiry time indicator if the time indicated by the stored expiry time indicator is before the current time. receiving, at a client system from a user device, a first access request comprising a first instruction to access a protected resource stored at a resource system; transmitting, from the client system to an authorisation system, a token request for an access token to be used for accessing the protected resource, in response to the first access request; receiving, at the client system, an access token in response to the token request, the access token having a corresponding time to expire indicative of a time at which the access token will not be valid for obtaining the protected resource from the resource system; in response to the token request, receiving and storing at the token storage unit an expiry time indicator corresponding to the received access token's time to expire, wherein the expiry time indicator is indicative of the time at which the corresponding access token will not be valid for obtaining the protected resource from the resource system, wherein storage of the access token at the token storage unit is maintained until a time after the time indicated by the expiry time indicator; transmitting the access token from the client system to the resource system and, in response, receiving the protected resource; storing the access token at a token storage unit of the client system; receiving, at the client system from the user device, a second access request comprising a second instruction to access a protected resource stored at the resource system; and in response to the second access request, transmitting the stored access token to the resource system for receiving the protected resource, characterized by performing, by the client system, token storage maintenance steps, wherein the token storage maintenance steps are executed according to a predetermined schedule that defines a time interval between adjacent executions of the token storage maintenance steps, the token storage maintenance steps comprising: comparing the time indicated by the stored expiry time indicator with a current time; and deleting the access token corresponding with the stored expiry time indicator if the time indicated by the stored expiry time indicator is before the current time. comparing the time indicated by the stored expiry time indicator with a current time; and deleting the access token corresponding with the stored expiry time indicator if the time indicated by the stored expiry time indicator is before the current time.