The governing body announced last week that Knights and Championship title rivals Ealing Trailfinders had failed to meet the minimum standards criteria for promotion.
Knights have said they will appeal against the ruling.
The RFU’s ruling sparked a big response on social media. Here’s how Knights and rugby fans in general responded to the news…
@MichaelCasey91: In recent years Castle Park has hosted multiple womens & age grade Six Nations fixtures. Has one of the finest playing surfaces in the sport. Denied opportunity for promotion due to lack of seats, how many @premrugby sides don’t average 10k gate?
@oliverstedman: What is the point in the league then? No promotion, no relegation. Just glorified friendlies and a tick box exercise for the RFU.
@GaeWilliams: Don't the RFU understand they are ruining the dreams of hard working and ambitious players by refusing Doncaster and Ealing the right to be promoted? What's the point of playing in the Championship if you can't go up? The rest of the season is worthless.
@SamHLWilliams: Ridiculous. The RFU are blocking aspiring clubs from progressing to the next level who have earned their spot to compete there.
@theboromassive: Castle Park is a terrific venue. Much better than some prem grounds. Shocking decision.
@sportslaw_uk: That's a terrible shortsighted decision - leaves Yorkshire without a club in the Premiership and a huge void for rugby union in the area. @RFU why do you continue to make such poor decisions?
@markjohnsonhome: As a @BristolBears fan, I am appalled by this. If Donny win the Championship they should go up, simple as that. I remember our play off games against you and what a scare you gave us. You deserve your place in the Prem and the RFU are failing the game.
@GDNCLE: As a @FalconsRugby supporter, this is shocking. When we end the season at the foot of the table, as I am expecting, we should be relegated. This ringfencing removes the need for us to win a match ever again.
@JohnHar70493442: Not sure how we 'grow the game' if we continue to restrict promotion between leagues.
@Pontemiller: RFU only want certain teams, with a certain size of fan base playing in the top flight. Best of luck in your search for justice.
@nicsue44: Good luck with your appeal. Promotion and relegation are good for rugby.
Andy_Wilson68: As a Bedford Blues fan I wish you all the best in your appeal. We must stop the Premiership becoming a closed shop.
@andywells3: Shameful decision. Yet other established clubs can have substandard stadiums and capacities that barely hit 10,000.
@tonywalks_1965: What’s the point of them applying if they knowingly don’t meet the criteria?