Use onClientClick
like this
<asp:Button ID="btnSubmit" OnClick="btnSubmit_click()" OnClientClick="return check()"
runat="server"/>
<script type="text/javascript">
function check() {
if (confirm('Are you sure you want to submit ?')) {
// do further form validation here..
return true;
}
else {
return false;
}
}
</script>