They are urging the UN's Economic and Social Council (ECOSOC) to reverse a decision by another UN committee in February 2017 not to grant Christian Solidarity Worldwide (CSW) official UN accreditation.
ECOSOC is due to discuss the matter at a meeting next Tuesday.
The letter states: "As one of the few non-governmental organisations advocating for freedom of religion or belief for people of all faiths and none at the UN, the hampering of CSW's ability to participate fully in UN processes also undermines the promotion of this right within the UN system at a time when religious narratives are increasingly impacting the political and social order."
Please pray CSW would be given UN accreditation (ECOSOC consultative status), as our appeal is considered. pic.twitter.com/AD5vwtviCD
— CSW_UK (@CSW_UK) April 11, 2017
CSW says the state would give them independent access to "key human rights advocacy platforms at the UN", including "the ability to organise side-events independently at fora such as the UN Human Rights Council (HRC) in Geneva".
The UN Committee on Non-Governmental Organisations decided two months ago to reject CSW's application for consultative status after repeatedly deferring it since 2009.
Nobel Laureates Jose Ramos-Horta, the UN Special Rapporteur for Freedom of Religion or Belief, Dr Ahmed Shaheed and the EU Special Envoy on Freedom of Religion or Belief outside the European Union, Ján Figel have also signed the letter.
The joint letter stresses that "This deferral and denial of applications threatens democracy within the UN system by undermining accountability, transparency and inclusiveness."
CSW's Chief Executive Mervyn Thomas said: "We are deeply grateful to the signatories of these letters and for the continuing support of the UK Mission to the UN for our application.
"The repeated deferral and ultimate denial of our application by the NGO Committee raises serious questions about its commitment to fulfilling its mandated responsibility to facilitate civil society participation at the UN, and impedes us from contributing fully to the promotion of freedom of religion or belief within the UN system at a time when religious narratives are increasingly impacting global affairs."