Select Related Entries from a specific category in CP I'm creating a site which has a products section.

I have created an entries section called 'Products'. 

I have set up some categories, i.e. 'Category One, 'Category Two', 'Category Three'

I have also created a related entries field so I can related products to each other.

When the client adds a new product in 'Category One', they should only be able to relate entries that are in 'Category Two' or 'Category Three'.

Is there a way that I can restrict the results returned in an entries field in the Control Panel to a specific category?
There is no native functionality for this at the current time.

You could write a plugin to extend the Products field-type (which itself extends the base entries field-type) - to provide this filter functionality.  That is you'd add a categories chooser to the field type and then filter the available products using that category in your plugin.

https://pluginfactory.io would get you started on the plugin scaffolding, and I'd look at /craft/plugins/commerce/fieldtypes/Commerce_ProductsFieldType.php and the native Craft categories field type to get started.