It comes after the 16 member clubs voted 8-8 in a secret ballot last night on a proposal to accept the trio. But it is a one-season arrangement only and talks have now started on a pyramid restructure to address concerns about B team participation.
The Scottish FA has confirmed that the three guest clubs are required to field seven players eligible for Scotland national teams in the starting line-up. Each guest club will also have to “provide funding to help develop the league and its clubs”.
The national governing body will also lead on a review of the pyramid structure, with recommendations proposed ahead of the 2023/24 season.
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The governing body said in a statement: “In addition, the respective league bodies have agreed to participate in the Scottish FA's pyramid working group discussions aimed at providing a longer-term solution to optimise the structure and aid the transition of talented players between the ages of 17 and 21.”
Hearts B are expected to play home matches at Ferguson Park, Rosewell, home of East of Scotland League club Whitehill Welfare. Along with Celtic and Rangers, they will pay an entry fee of £40,000, an increase on the £25,000 that Celtic and Rangers were charged last season.
Member clubs had previously dismissed the idea, initially voting to permit only two guest teams in keeping with last season and then rejecting a 19-team league when three expressed an interest. But when faced with an ultimatum that it was all three or none, the issue was put to another vote last night.
Cowdenbeath and Tranent were able to vote for the first time as new members, with Bonnyrigg Rose and Vale of Leithen having stepped down after promotion and relegation.
Clubs had sought and received additional assurances at last week’s AGM, where they agreed to tighten up rules and regulations on fixture rescheduling, player eligibility and declaring a champion in the season ahead.
The Celtic, Hearts and Rangers B teams will be under-20s and will be ineligible for promotion and relegation, but results against them will count. Rangers and Celtic B teams finished second and third behind Bonnyrigg in 2012/22 and the SFA has pledged that the member club who finish highest in the table will progress into the pyramid play-offs for a place in League 2.
There are concerns that a 36-game season and an odd number of teams will cause fixture congestion, but the tightening up of regulations and the additional income that Celtic, Hearts and Rangers could bring may well have persuaded some clubs.
Along with Celtic and Rangers, Hearts feel the gap between the Club Academy Scotland Under-18 League and the first team is too large but would prefer to keep their own young players and give them regular matches rather than send them all out on loan.
But the decision to allow three B teams to join after refusing to increase the number of promotion places into the Lowland League clubs has angered many clubs in the three tier six divisions below.
The East, West and South of Scotland League champions will again have to enter a three-way play-off for one promotion place at the end of forthcoming season unless a restructure is put in place beforehand. The pyramid working group has reconvened and the Edinburgh Evening News has learned that it aims to have new proposals about how best to accommodate colts teams in place by the end of this year.
SFA chief executive Ian Maxwell said: "We are delighted that the Lowland League clubs have approved the participation of three guest clubs for the forthcoming season. The Scottish FA supports the principle of elite player development and particularly providing opportunities for talented players in the key transition years from 17 to 21.
“The Scottish FA will oversee a discussion in parallel to propose and implement a long-term plan with the objectives of optimising the pyramid, which has been a resounding success in Scottish football, harnessing the player pathway and accelerating the development of the country’s most talented young players through the national youth teams to the full international stage.”
Lowland League chairman Thomas Brown said: "We were always open to the inclusion of guest clubs for a further season given the successful player development outcomes from last year's pilot. We want to be a modern and progressive league and we are excited to be playing our part in the ecosystem that develops Scotland’s talent.
"Importantly, we now look forward to working with the Scottish FA, the SPFL and our friends in the pyramid to create some new solutions to some existing challenges which will provide a bright future at all levels of our game."
Neil Doncaster, the SPFL chief executive, added: “The SPFL will commit our energies to a process that will explore options and, hopefully, allow us to move forward various aspects of our game for the benefit of our 42 member clubs and Scottish football as a whole."