Shaun Edwards believes both he and Wales would have benefited had he been granted permission to work with Toulon.
Wales defence coach Edwards had been expected to take up a part-time position with the Top 14 club, after receiving the blessing of head coach Warren Gatland.
However, the Welsh Rugby Union (WRU) duly released a statement confirming the move had been blocked, stating: "The scope of the opportunity has changed and [it] would represent an unacceptable compromise."
In an interview with WalesOnline, Edwards said: "I want Welsh fans to know the truth, which is that I would never prioritise anything over the Wales team.
"Warren had said I could go. He and I think I could learn from working with those great players they have at Toulon and bring some of the knowledge gained back to Wales.
"I felt it would have been a vehicle to develop myself as a coach because you’re always learning and bring the findings back. I do think Wales and I would have benefited from it.
"But the union has said I can't go, so I can't."
Explaining the terms of the proposed deal, Edwards added: "I would have only popped over on a consultancy basis. It would have been about a dozen sessions over the full season."
"Toulon have never practised defence before but after being hammered by Wasps in last season's Champions Cup they decided to do so.
"I feel I would have improved as a coach working with star names like Duane Vermeulen, Ma'a Nonu and Bryan Habana.
"Habana is the player I most of all wanted to work alongside because left wing has been a defensive weakness for us with Wales and is an area we need to improve."