It work for me, try with this.
User Control MarkUp
<%@ Control Language="C#" AutoEventWireup="true" CodeFile="Control1.ascx.cs" Inherits="Control" %>
<asp:Label ID="lblTest" runat="server" Text="This is a test"></asp:Label>
Default.aspx Page Code Behind
const string path = "~/Control1.ascx";
HtmlGenericControl div;
protected void Page_Load(object sender, EventArgs e){
div = new HtmlGenericControl("div");
div.Attributes.Add("runat", "server");
var userControl = LoadControl(path);
if (userControl != null)
div.Controls.Add(userControl);
this.form1.Controls.Add(div);
}