JavaScript not working inside web user control

I hava a page default.aspx having master page abc.aspx and web user control login.ascx i want to call perform simple alert function on login button but not on client click event. For e.g

Protected Sub BtnMyIpLogin_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles btnMyIpLig.Click

'Some Validation part here for user login

'then my if condition like

If flag_alert = 0 Then                 Page.ClientScript.RegisterStartupScript(Me.GetType(), "btnflag", "alert('Your Download / View Limit Exceed More Than 90%');", True)             End If

but it is not working.........................

Plz Help