How do I prevent Excel from locking files by default?

When I double-click on a CSV file on a network share, the file is opened in Microsoft Excel (which is what I want). However, Excel assumes that I'm going to modify the file, and that everyone else is too, and so puts a lock on it.

In practice I very rarely actually want to modify these files, merely read from them. And if I have the file open in an Excel window in the background, it stops anyone else from opening the same file.

I am aware that I can manually open a file as read-only from the File -> Open dialog within Excel. However I almost always open files by double-clicking on them in Explorer (or Outlook, for attachments). Is it possible to change the file association so that the default handler for CSV files is "Excel in read-only mode"? Is there a command-line argument that I can use in the Open With... dialog to achieve this?

Or more bluntly - when I am looking at a CSV file in Windows Explorer, is there an easier way to open it read-only than starting up Excel myself, selecting File -> Open, choosing "read only" from the dropdown, manually navigating to the same folder in the hierarchy, and then opening the file?

(I am happy to have to jump through hoops on the rare occasions that I want to modify and save a file.)

I was able to reach a satisfactory conclusion to this by adding the following keys to my registry:

[HKEY_CLASSES_ROOT\Excel.CSV\shell\Open_in_read_only_Excel]
@="Open read-only in Excel"

[HKEY_CLASSES_ROOT\Excel.CSV\shell\Open_in_read_only_Excel\command]
@="\"C:\\Program Files (x86)\\Microsoft Office\\Office12\\EXCEL.EXE\" /r \"%1\""

These create an entry on the right-click shell menu in Windows Explorer called "Open read-only in Excel". When selected, this launches Excel with the /r flag (as per the command in the second key), which opens the file in read-only mode.

This is not perfect - I would rather that this were the default action for a CSV file, and that a context option was needed to launch in read-write mode. However it is a vast improvement on the situation before.

Here's the equivalent of Andrzej Doyle's answer for Excel 2013, which uses DDE, and also sets "Open (read-only)" as the default action:

Windows Registry Editor Version 5.00

[HKEY_CLASSES_ROOT\Excel.CSV\shell]
@="OpenReadOnly"

[HKEY_CLASSES_ROOT\Excel.CSV\shell\OpenReadOnly]
@="Open (read-only)"

[HKEY_CLASSES_ROOT\Excel.CSV\shell\OpenReadOnly\command]
@="\"C:\\Program Files\\Microsoft Office 15\\Root\\Office15\\EXCEL.EXE\" /dde"

[HKEY_CLASSES_ROOT\Excel.CSV\shell\OpenReadOnly\ddeexec]
@="[open(\"%1\" /ou \"%u\",,1)]"

[HKEY_CLASSES_ROOT\Excel.CSV\shell\OpenReadOnly\ddeexec\topic]
@="system"

(Yes, it's the ,,1 that makes the file read-only. How obscure!)