Top officials at the Rugby Football Union have been accused of betraying and misleading the game by the former England international who has been helping to negotiate the sport’s future below the Premiership. Simon Halliday, part of England’s 1992 Five Nations grand slam-winning side, has also called for a review into the “significant” failings of some RFU executive directors.

Halliday, who was chair of European Professional Club Rugby for seven years until 2021, has latterly been representing Championship clubs seeking greater funding and firm guarantees from the RFU over promotion and relegation. In an excoriating letter sent to the RFU’s chair, Tom Ilube, seen by the Observer, he alleges the existing tier 2 clubs “have been stalled, misled and misinformed” and warns recent poor governance “threatens the game” in England.

Related: Francis Baron: ‘The RFU is not expecting to make a profit for almost 10 years. You can’t go on like this’

In particular, Halliday alleges RFU executives have reneged on assurances given at a council meeting in mid-June to talk further about softening the Premiership minimum standards criteria to make it more feasible for sides to be promoted to the top tier. “With the help of our own legal advisers … I made it quite clear that some matters were not agreed and needed further debate,” Halliday wrote. “This was signed off by the RFU executive and I have the written evidence, as well as that of our lawyer. Since then, we have been stalled, misled, misinformed and there is no sign of a more progressive approach to this fundamental part of the game.”

A two-leg playoff between the Championship’s top side and the Premiership’s bottom team had been hailed as a means of retaining promotion and relegation. But a crucial tweak to the small print now requires sides lacking an existing ground with a capacity exceeding 10,000 to have advance planning permission and financial assurances in place guaranteeing their stadium expansion work will happen within four years, in effect leaving every Championship club, bar ninth-placed Doncaster, unable to go up.

Halliday, who stepped down in August as chair of the Championship board, has urged Ilube either to “make a statement clarifying that promotion and relegation is dead” or to “conduct an immediate review” before the 1 December deadline for applying for a facilities audit.

“You potentially have no club which can be sustainably promoted,” wrote Halliday. “How is this acceptable? It is the RFU who is expected to look after the whole game. Instead, you are alienating the very clubs … for whom you are responsible.”

The letter also claims that efforts by the Tier 2 Board to negotiate fair and reasonable criteria for ground capacity were circumvented by decisions taken at a Professional Game Board meeting that was allegedly not quorate. Halliday believes “the legality of what has taken place is clearly questionable” and says aspiring Championship clubs have been left in “an unacceptable position” by the RFU. “Your executive directors have failed in significant fashion to take care of this process,” he told Ilube.

“Recent actions by the RFU legal team seem to have been [about] obfuscating and protecting the status quo which is clearly not consistent with [RFU] Council decisions. This threatens the game in our country given its seriousness. Further, the commitment by Bill Sweeney and the [RFU] executive to discuss in good faith the funding gap between the promoted club and the existing Premiership clubs has not been fulfilled despite numerous requests.”

The RFU is already under fire on several fronts, with significant year-end financial losses due to be reported imminently and the national team having lost seven of their past nine Tests. There are also fears the financial situation at Twickenham may frustrate a plan by the Rugby Players’ Association to extend much-needed welfare support to tier 2 players.

In response the RFU stressed that a new Tier 2 Board with an independent chair was now in place. “It is through this board that all matters relating to rugby’s second tier are raised and managed,” said a spokesperson. “The board members are working collaboratively to deliver a reimagined tier 2 from next season.”