The Grand Chamber of the European Court of Human Rights is set to rule on an inheritance case concerning the application of sharia law to a dispute between Greek citizens who are Muslims.
The ruling will determine whether sharia law can have supremacy over a member state's domestic law.
The court granted Christian Concern permission to intervene in the case of Molla Sali v. Greece.
According to Christian Concern, Ms Molla Sali inherited the entire estate of her husband when he died under the terms of a will that he had drawn up in accordance with Greek law.
Two sisters of her husband are claiming that since her husband was a Muslim, the inheritance should be allocated according to sharia law, adjudicated by the mufti.
There are provisions for sharia law to be applied to Greek nationals who are Muslims.
The charity said the case is important because "a ruling at this level... could have dire effects on the Council of Europe's 47 member states, including the United Kingdom." Also, rulings of the Grand Chamber of the European Court of Human Rights cannot be appealed.
Christian Concern said it will highlight in court that Sharia law is out of line with European law.
It said in a statement: "A network of sharia courts is operating across the country, creating a de facto parallel legal system which undermines the fundamental principle of one law for all.
"[The intervention] also highlights the discriminatory nature of sharia law which means, for example, that non-Muslims cannot inherit from Muslims, and females only inherit half of what is due to males."
It added: "Sharia law is fundamentally incompatible with human rights, and our intervention explains that the Universal Islamic Declaration of Human Rights restricts freedom of speech and freedom of religion, as does the Cairo Declaration on Human Rights.
Bishop Michael Nazir-Ali, whose expertise was used in the intervention, commented: "Sharia law is based on the inequality of certain kinds of persons whereas western public law is based on the equality of all persons.
"Muslims, like people of any other faith, are free to practise their faith, but because sharia is incompatible with western public law, it should not be given supremacy over the domestic law of western states.
"The specific case in this part of Greece is based on an historic agreement that should not be applied to the rest of Europe or taken as a precedent for the rest of Europe."
The case is set to be heard in the Grand Chamber on 6 December 2017.