The Exiles won the Championship title on Wednesday night, beating Cornish Pirates 66-41 over two legs, but the RFU decreed the club had failed to meet the minimum standards to be allowed into the Premiership.
London Welsh chairman Bleddyn Phillips said: “We shall be lodging an appeal in the very near future.
“In parallel we are pursuing all other available opportunities to make sure we can take our rightful and deserved place in the Aviva Premiership.”
The final decision to appeal was taken after meeting of the London Welsh board yesterday.
London Welsh were informed last Wednesday, just hours before the first leg of the Championship final, that they would not be eligible for promotion if they won the title.
The RFU board had ratified the findings of an independent audit which highlighted primacy of tenure of London Welsh’s nominated home ground – the Kassam Stadium in Oxford – as a key issue.
Phillips, a partner in the law firm Clifford Chance, described the RFU’s decision as “inequitable, unreasonable and unfair”.