Can Modal Popups be used inside an AJAX tab container

If any one can please help me with this, I'd be really grateful

I'm currently having an issue with the following Modal Pop up, that's inside a Tab Panel/ Tab container

<cc1:ModalPopupExtender ID="ModalPopupWarning" runat="server" 
                        PopupControlID="PanelWarning" 
                        TargetControlID="btnShow"
                        CancelControlID="btnClose" 
                        dropshadow="true" 
                        BackgroundCssClass="modalBackground"></cc1:ModalPopupExtender><asp:Panel ID="PanelWarning" runat="server" CssClass="modalPopup" 
                        Width="668px">     <div>            <h3><asp:Label ID="lblWarning" runat="server" Text="Label">Warning</asp:Label></h3>     <div class="modalFont">     
                                Please confirm that this approval has been granted by checking the box below 
                                before proceeding.<br /><br />  <asp:Label ID="lblApprovalObtained" runat="server" Text="Label">Approval Obtained:</asp:Label> <asp:CheckBox ID="chkApprovalObtained" runat="server" /></div><br /><asp:Button ID="btnNext" runat="server" Text="Next" onclick="btnNext_Click"  />   <asp:Button ID="btnClose" runat="server" Text="Close" /></div></asp:Panel>  

However, the btnClose takes me to the next panel 'ModalPopUpApproval'  and notbtnNext as intended, this is based on the 'chkApprovalObtained' as below:   

'Shown on modal popup warning
    Protected Sub btnNext_Click(ByVal sender As Object, ByVal e As EventArgs) Handles btnNext.Click
        If chkApprovalObtained.Checked = True Then
            ModalPopupApproval.Show()
            'resets approval type - incase users selects it, then cancels and returns.
            ddlApprovalType.SelectedValue = "1"
            lblApprovalObtained.ForeColor = Drawing.Color.Black
        Else
            ModalPopupWarning.Show()
            lblApprovalObtained.ForeColor = Drawing.Color.Red
        End If
    End Sub

Am I missing something because this is inside a Tab Panel?