Leading Zero Issue when exporting excel from grid

In the below code i am export excel from grid when export the leading zeros get deleted.I want to maintain the leading zero.Please help me to solve the issue.

HtmlForm form = new HtmlForm();
                    Response.Clear();
                    Response.Buffer = true;
                    Response.Charset = "";
                    Response.AddHeader("content-disposition", string.Format("attachment;filename={0}", "Supply/DemandReport.xls"));
                    Response.ContentType = "application/ms-excel";
                    StringWriter sw = new StringWriter();
                    HtmlTextWriter hw = new HtmlTextWriter(sw);
                    gdSupplyorDemand.AllowPaging = false;
                    gdSupplyorDemand.HeaderStyle.ForeColor = System.Drawing.Color.Black;

                    form.Attributes["runat"] = "server";
                    form.Controls.Add(gdSupplyorDemand);
                    this.Controls.Add(form);
                    form.RenderControl(hw);
                    string style = @"<style> .textmode { mso-number-form at:\@;}</style>";
                    Response.Write(style);
                    Response.Output.Write(sw.ToString());
                    Response.Flush();
                    Response.End();