PHNOM PENH (Reuters) - The European Union has suspended funding for Cambodia s 2018 general election because the vote cannot be credible after the dissolution of the main opposition party, according to a letter sent to the national election committee on Tuesday. The Cambodia National Rescue Party (CNRP) was dissolved by the country s highest court last month at the request of the government of Prime Minister Hun Sen after the arrest of opposition leader Kem Sokha for alleged treason.  An electoral process from which the main opposition party has been arbitrarily excluded cannot be seen as legitimate,  the letter said.  Under these circumstances, the European Union does not believe there is a possibility of a credible electoral process.  Some Western donors have condemned the CNRP dissolution as the most serious blow to democracy since an international peace deal and U.N.-run elections in the early 1990s ended decades of war and the Khmer Rouge genocide that killed at least 1.8 million Cambodians in the 1970s. A government spokesman said the EU decision would not affect the election, in which Hun Sen is expected to extend more than three decades in office that make him the world s longest serving prime minister.  This is their will,  spokesman Phay Siphan told Reuters on Tuesday.  We have our own money.   The European Union and Japan are the biggest donors to Cambodia s election commission, which is in theory independent. Asked if Japan would also suspend aid for the election, the Japanese embassy said it would continue to give assistance for electoral reform and would monitor the situation closely.  It is of utmost importance to have next year s national election reflect the will of the Cambodian people,  Hironori Suzuki, a counselor at the embassy, told Reuters by email. The United States last month said it would suspend funding for the election. It later said it would impose visa sanctions on those involved in the government s actions to undermine democracy.  Kem Sokha, leader of the CNRP, was arrested for allegedly plotting to overthrow the government with U.S. help. He has rejected the accusation as a political ploy. The European Union in October warned Cambodia it could face EU action over duty-free access it enjoys under a deal for some of the world s poorest countries if the nation s human rights situation deteriorates further. Cambodia s biggest donor and investor is now China, which has voiced support for the opposition crackdown, saying it supports government efforts to ensure stability and economic development. 