If we are using HTTP-GET protocol to invoke the Web Service, ReturnFormatter property must be set to XmlReturnReader and ParameterFormatter property must be set to UrlParameterWriter in HttpMethodAttribute attribute.
This attribute is required to indicate the types that serialize parameters sent to a Web Service method and read the response from the Web Service.
In our case it would be as follows:
[System.Web.Services.Protocols.HttpMethodAttribute(typeof(System.Web.Services.Protocols.XmlReturnReader),
typeof(System.Web.Services.Protocols.UrlParameterWriter))]