The home secretary, Theresa May, has come under renewed criticism over the independent inquiry into child sex abuse as controversy rages over the issue of survivors being appointed to its advisory panel.

Michael Mansfield QC has lent his name to a campaign to force a U-turn on the proposed membership criteria. At one stage Mansfield was considered as a possible head of the inquiry.

More than 200 survivors, whistleblowers and child protection professionals have expressed dismay at the declaration by the inquiry’s new head, Lowell Goddard, that survivors lack sufficient objectivity to be full quasi-judicial members. In an open letter to May, they point out British law does not bar child abuse victims from acting as judges or jurors on abuse cases. They say stigmatisation of survivors for lacking objectivity, before the inquiry has even started work, could doom the inquiry to failure.

The last inquiry into child sex abuse was scrapped by May following a series of scandals, including links between two chosen heads and those publicly accused of being involved in abuse.

Phil Frampton, a survivor who signed a joint letter to May and to Ben Emmerson, counsel to the inquiry, said: “Theresa May cannot expect to be believed when she says she wants survivors at the heart of the inquiry whilst barring them as not fit to make decisions.