It is being claimed that a senior consultant at St James' in Dublin has outlined serious concerns about the proposed new National Children's Hospital.
The founder of the Jack and Jill Foundation Jonathan Irwin said he had received an anonymous letter containing "horrifying things that are under the carpet".
The design team for the new facility at the Dublin 8 campus will be announced this morning.
Last month, Mr Irwin revealed he had received two other letters by people believed to have a wide knowledge about the facility, in which they raised concerns about its proposed location.
Jack and Jill CEO Jonathan Irwin said these people did not feel they could raise their concerns publicly.
"(The letter) arrived on my desk yesterday on St James' Hospital writing paper," he said. "He apologises for not being able to reveal who he is."
The letter-writer says that members of his profession are prevented from "coming out front" by the fear of suffering "wicked financial consequences in their career or from the HSE".