The Prem has been reduced to 10 sides after Wasps, Worcester and London Irish were forced to fold during the 2022-23 season.
Traditional promotion and relegation to and from the top tier has been scrapped from the start of the 2026-27 season after landmark changes to English rugby were voted through.
Instead of earning a play-off shot at replacing the Prem's bottom side by finishing top of the second-tier Champ and meeting the top flight's minimum standards, clubs will instead apply to join and be judged across a range of criteria.
Bids will be assessed on standard of play, finances, investment potential, infrastructure and how their geographical base could help spread the Prem's appeal, among other factors.
The league hopes to initially expand to 12 teams for the 2029-30 season with a "long-term ambition" to grow further.
Conditions were initially set that any team wanting to play in the Prem will need to complete a campaign in the Champ before being considered.
A joint Prem Rugby and RFU statement said: "Prem Rugby and the RFU are committed to expanding the English top-flight, with a provisional target season to grow to 12 teams by 2029-30.
"The Prem is an attractive league and responsibility for its expansion sits with the Men's Professional Rugby Board (MPRB), which has established detailed entry criteria that any prospective club would need to satisfy.
"Resources are currently focused on supporting English clubs throughout the pyramid who aspire to join the league."
The RFU would have to approve Welsh teams moving into the league due to it becoming a cross-border competition.
Wales have four professional sides, Ospreys, Cardiff, Scarlets and Dragons, that currently play in the 16-team United Rugby Championship (URC) which includes teams from Scotland, Ireland, Italy and South Africa.
Welsh rugby's governing body, along with the other national unions, are stakeholders in that league.
While English bosses have revealed their intentions were on welcoming English clubs, the WRU released a more non-committal statement where they did not rule out continuing the English interest.
"Welsh men's professional teams compete in the URC and the WRU is committed to providing four teams to the URC until the end of the 2027-28 season," it read.
"By the end of this season, the WRU will have determined the future operating model for the men's game in Wales.
"The WRU will continue to work with other shareholders in the URC on future Welsh representation in the competition.
"The WRU keeps under review the competition opportunities which may be available across the game and could be in the best interests of Welsh rugby.
"The idea of Anglo-Welsh competitions is something often debated by Welsh rugby fans.
"The WRU understands the appeal of the English top-flight, which is why we are pitching for two of our women's teams, Brython Thunder and Gwalia Lightning, to be included in Premiership Women's Rugby."
New WRU chair Lyndon Faulkner has recently stated he is fan of any potential Anglo-Welsh competition.
The WRU posted an £18m loss for the latest financial year - compared to a £7.2m loss the previous year.
The union says it is already predicting a similar loss next year, and say the figures back up its case that four professional regions are no longer sustainable.
WRU chief financial officer Gavin Marshall said they could only spend £20m on professional rugby, compared to the £26m it contributes now.
However it is understood the Prem "P-share" - effectively the up-front cost to enter the league - is valued at £20m.
The "P-share" secures a percentage of central income and voting rights and the requirement to fund a Premiership Women's Rugby programme.
How any new Anglo-Welsh tournament ties in with WRU plans to cut a professional side by 2028 remains to be seen.
The governing body has stated it will press ahead with these plans with three licences awarded, one in the capital, one in the east and one in the west.
Ospreys and Scarlets are set to fight to be the side in west Wales, with Cardiff, currently owned by the WRU, and Dragons - an independently-backed business - safe under current proposals.
As it stands, a rugby summit has been called for December with a final decision expected on which club will be culled to be made in the summer of 2027.
But Faulkner has also promised to review the three-team proposal as the uncertainty in Welsh rugby remains.
Source: BBC Sport · Gareth Griffiths, Chris Jones
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