Most of Wales' England-based players will not be allowed to play in Wales' first World Cup warm-up game against Ireland on 8 August.
Gloucester's Richard Hibbard and Bath-bound Rhys Priestland are unavailable until 14 August, which also rules them out of training camps in July.
But George North is unaffected as he has a clause in his Northampton deal.
World Rugby's laws state clubs are not obliged to release players until 35 days before the tournament starts.
Premiership Rugby's policy "is to release players within the rules".
Bath prop Paul James is rejoining Ospreys at the end of the season and will be available for Wales' preparations under coach Warren Gatland.
Player release rules are set out in World Rugby's regulation nine, which states countries can only expect to see their players five weeks ahead of the start of the tournament.
That takes place on Friday 14 September when hosts England face Fiji at Twickenham.
England, Wales and Australia are in Pool A along with Fiji and Uruguay.
|2015 Rugby World Cup pools|
|Pool A: Australia, England, Wales, Fiji, Uruguay|
|Pool B: South Africa, Samoa, Scotland, Japan, USA|
|Pool C: New Zealand, Argentina, Tonga, Georgia, Namibia|
|Pool D: France, Ireland, Italy, Canada, Romania|
In July Wales will have training camps in Qatar and Switzerland.
The Welsh Rugby Union has an agreement with Wales' four regions via umbrella body Pro Rugby Wales that ensures players' extended release.
However, Newport Gwent Dragons, Cardiff Blues, Ospreys and Scarlets are under no obligation to release foreign players early.
The leading English clubs' decision means players from other World Cup nations such as Fiji, Tonga, Samoa and Argentina will also not be free to begin World Cup preparations until 14 August.
Leading French clubs can also take the approach of England's top teams.
That could cause difficulties for Jamie Roberts, Luke Charteris and Mike Phillips (all Racing Metro), plus Jonathan Davies (Clermont Auvergne) and Leigh Halfpenny (Toulon).
Priestland has admitted moving from Scarlets to Bath could harm his Wales career.
Hosts England have a deal with the Aviva Premiership clubs which ensures release for their World Cup build-up from June.
Australia and holders New Zealand pick only home-based players so will be unaffected while Ireland have no leading players employed in England.