Wales coach Warren Gatland will have access to his Grand Slam winners for 13 days before their opener against Scotland at Murrayfield on 8 February after achieving player access he wanted for the RBS 6 Nations.
The major boost to Wales' hopes of retaining the Six Nations title comes after an agreement between the Welsh Rugby Union and the Welsh regions.
Wales also have first call on the players for the entire competition, even though there are two weekends of regional matches during the tournament.
The regions agreed to the terms after meetings at the start of this week.
England Under-18 head coach John Fletcher insists his young side will be better for the experience after beating Italy in the bronze final to round out their 2015 Rugby Europe U18 Championship campaign in style.