Google returning 403 Forbidden

My webservice makes a request to maps.google.com but returns exception System.Net.WebException with 403 Forbidden message

But the URL usually runs via browser. What can it be?

[WebMethod]
public string teste()
{
    string url = @"http://maps.google.com/maps/api/directions/xml?origin=-28.2773357297022,-52.7852053516473&destination=-19.7779008,-47.9249145&mode=driving&waypoints=-26.8727562,-49.1010188|-25.530283,-49.2949339|-23.4841434977362,-46.7853496799363|&sensor=false&client=gme-meuID&signature=G3GSye-fxYY-GIDq-z2TIO8FI2A=";   
    HttpWebRequest _HttpReq = (HttpWebRequest)WebRequest.Create(url);
    _HttpReq.Method = "GET";
    var response = _HttpReq.GetResponse();
    StreamReader sReader = new StreamReader(response.GetResponseStream());
    return sReader.ReadToEnd();
}