A campaign group fighting cuts at the Friarage is considering a judicial review over the changes at the Northallerton hospital.
The Save The Friarage Campaign says it has sought legal advice from law firm Irwin Mitchell over the replacement of the A&E with an urgent treatment centre.
The change occurred on Wednesday and came after NHS bosses said they were unable to fill vacancies for anaesthetists to work at the hospital.
They said the change to an urgent treatment centre was needed to ensure the Friarage could still provide safe care.
Senior clinicians say the vast majority of patients who previously used the Friarage would continue to do so, with the urgent treatment centre able to provide 24/7 care for adults and children with more minor injuries and illnesses.
The group said in a statement issued today that they have been advised that the only way forward to ensure the changes have been made lawfully and were safe was to request a judicial review.
The group said it has sent letters to South Tees Hospitals NHS Foundation Trust and Hambleton, Richmondshire and Whitby CCG requesting documents demonstrating their “due diligence and decision-making process” ahead of the decisions.
The statement says: “The aim of doing so is to gain more transparency as to how this crisis has been allowed to arise and why these changes are the only solution being proposed.
“Since merging in 2002 the commitment and investment promised by South Tees Trust has not materialised and instead the hospital has faced continuous cuts in service provision at the expense of the local community it serves.
“We feel these most recent cuts are jeopardising the lives of the 120,000 individuals over our rural demographic and sentencing the Friarage to further decline which we believe will lead to further cuts in the future.”
The campaign group says the pubic meeting on Friday did not adequately address the “grave concerns” campaigners have have raised regarding the safety of the new model.
They add: “We question whether the South Tees Trust has performed a thorough assessment in relation to the changes and the significant impact they will have on the patients, relatives, staffing, ambulance service and surrounding hospitals.
“Within our legal challenge we are asking to see that these concerns have been considered and stringent contingencies are in place to ensure patient safety is not compromised.
“We believe that lives are being put at risk and that we must ensure that these changes are in the best interests of the people who rely on this wonderful hospital not the long-term ambition of the South Tees Trust.”