[Microsoft.Web.Script.Services.ScriptService()] - The type or namespace name 'Web' does not exist in the namespace 'Microsoft'

Hi
I'm trying to use the [Microsoft.Web.Script.Services.ScriptService()]  in my web service, but when I compile I receive the following error message:
Error 1 The type or namespace name 'Web' does not exist in the namespace 'Microsoft' (are you missing an assembly reference?)
Any suggestions how to fix this problem?
Thanks in advance!
Regards
Edward

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