London Welsh will "explore all possible avenues" as they continue their quest for Aviva Premiership status next season.
The Exiles have been told they do not meet minimum standards criteria to play in English rugby's top flight and will not be promoted even if they complete victory in the second leg of their play-off with Cornish Pirates.
The decision was confirmed by Rugby Football Union directors just six hours before London Welsh beat the Pirates 37-21 in the first leg in Penzance.
RFU directors ratified findings of a report carried out by independent auditors that identified various London Welsh failures, including not having primacy of tenure at their nominated Premiership ground - Oxford United's 12,500-capacity Kassam Stadium.
The Pirates did not choose to have an independent audit of their Mennaye Field ground, so were therefore ineligible for promotion.
Welsh, whose chairman Bleddyn Phillips is a partner with the law firm Clifford Chance, have 14 days in which to lodge an appeal.
The club's board of directors have responded to the RFU announcement by saying they will be "exploring all possible avenues off the pitch to ensure that if successful in winning the Championship, London Welsh will take its rightful place in the top flight of English rugby."
As things stand, though, Premiership bottom club Newcastle will remain among the elite next term with no promotion taking place from the second tier.
The London Welsh statement added: "London Welsh RFC would like to express its grave disappointment on learning of this decision, coming as it does just a few hours ahead of the first leg of the club's Championship final against the Cornish Pirates.
"The club firmly believes that it has met the minimum standards criteria for promotion to the Premiership, particularly as regards the issue of primacy of tenure, and reaffirmed its belief in this respect in a detailed communication to the RFU as recently as Monday May 21 highlighting its case for having satisfied the MSC.
"The club has already sought clarification on this point, the detailed reasoning behind why primacy of tenure was not satisfied (among other things) having not yet been given by the RFU.
"The club is especially disappointed in this regard as a firm binding agreement, subject only to promotion to the Premiership, has already been concluded with Kassam Stadium - a ground with facilities the club believes to be at least the equal of many other grounds already hosting Premiership rugby."
Newcastle propped up the Premiership by a point despite winning their final game of the 22-game regular league season against fellow strugglers Wasps earlier this month.
But the Falcons have now survived - barring any successful London Welsh appeal - and they can look forward to the 2012-13 campaign when Dean Richards will take charge as rugby director.
Falcons commercial director Duncan Edward said: "To be given the lifeline of playing in the Premiership next season and keeping top-class rugby in the region is unbelievable."
In a statement, RFU directors said: "The Rugby Football Union's board of directors has considered the findings of the independent audit report requested by London Welsh, which determines a club's eligibility for promotion to the Aviva Premiership in line with the minimum standards criteria (MSC).
"The report, carried out by independent auditors, found that London Welsh has not met the MSC for the Aviva Premiership.
"The independent auditors identified various failures, including not having primacy of tenure at their nominated ground. This states that a club must demonstrate that they can host home fixtures at the time stipulated by Premiership Rugby and/or the host broadcaster.
"The RFU board ratified the findings and agreed, should London Welsh win the RFU Championship final following the second leg on May 30, they would not be eligible for promotion and Newcastle Falcons would remain in the Aviva Premiership.
"Cornish Pirates did not choose to have an independent audit of the Mennaye Field and so do not fulfil the criteria for promotion should they win the final. As a result, there will be no promotion from the RFU Championship this season."
The minimum standards criteria are set by the Professional Game Board (PGB) - a joint group from the RFU, Premiership Rugby, RFU Championship clubs and the Rugby Players' Association.
But today's outcome will inevitably stir the hornet's nest issue of promotion and relegation between the Championship and Premiership, giving further ammunition to those who believe the Premiership is destined to become a closed shop.
London Welsh, founded in 1885, currently play at Old Deer Park in Richmond, which has a capacity of just under 6,000.