The UK’s “colonial approach” to the Chagos Islands may yet damage hopes that a British QC will be appointed chief prosecutor at the international criminal court in succession to Fatou Bensouda. Karim Khan is the current favourite for the position. But the UK’s refusal to recognise two adverse international court judgments on the ownership of the Chagos Islands, a British claimed territory in the Indian Ocean, has stymied plans for consensus to build around Khan for the post. Members of the UN’s Assembly of State Parties, the management oversight body of the ICC, have now agreed to hold a secret ballot in which Khan and two other candidates, from Ireland and Spain, are nominated. The Mauritius ambassador to the UN, Jagdish Koonjul, wrote on Monday to all the relevant UN delegations expressing his alarm at the UK’s approach to international law. He said that he was prompted to act after receiving news of a statement from Britain’s Ministry of Defence saying the UK would not act on a ruling by the UN maritime tribunal affirming that Mauritius – and not the UK – was sovereign over Chagos Islands. Koonjul told the Guardian: “75 years ago the UK was at the front of setting up the institutions of the UN, and the importance of compliance with the international rule of law, as well as the need to complete decolonisation. “We find it so disappointing that the UK has now taken a position that is 180 degrees opposite. It makes it very hard for the UK to claim a right to run international bodies like the ICC. Its soft power is so weakened.” The new chief prosecutor will be a critical figure in the years ahead as the ICC tries to maintain credibility and prestige by bringing more cases to successful prosecution – as well as navigating sensitive issues such as jurisdiction over war crimes in Palestine. A British head of the ICC would also be a diplomatic boost for the UK in a post-Brexit world. Mauritius claims the UK has not accepted two international tribunal rulings on ownership of the islands, including one issued last month by the UN’s maritime tribunal. Diego Garcia, one of the islands in the archipelago, is used as a US military base. The special chamber of the international maritime court (International Tribunal for the Law of the Sea) on 28 January found the UK had no sovereignty over the Chagos islands, saying a previous advisory ruling made by the International Court of Justice (ICJ) did have legal effect and that “while the process of decolonization has yet to be completed, Mauritius’ sovereignty over the Chagos archipelago can be inferred from the ICJ’s determinations”. Koonjul referred to a new parliamentary written answer from defence minister James Heappey in which he asserted the maritime court ruling had no impact on the UK, since the case had been taken by Mauritius against the Maldives. In the answer Heappey wrote: “We have no doubt about our sovereignty over the territory of the British Indian Ocean Territory, which has been under continuous British sovereignty since 1814. “Mauritius has never held sovereignty over the Archipelago, and we do not recognise its claim. We have made a long-standing commitment to cede sovereignty of the territory to Mauritius when it is no longer required for defence purposes. We stand by that commitment. Owing to the UK’s sovereignty over the territory, the prevailing laws and regulations within its 12 nautical mile territorial sea are those enacted in governance of a British overseas territory. The domestic laws and regulations of Mauritius do not apply.” The shadow foreign secretary, Lisa Nandy, in a letter to the foreign secretary, Dominic Raab, said the UK position “is damaging to Britain’s reputation, undermines your credibility and moral authority and sets a damaging precedent that others may seize upon to undermine UK national interests, and those of our allies, in other contexts or maritime disputes”. She added: “I am also troubled to learn that the government’s position on the Chagos Islands has been met with serious concern by international allies who have long looked to the UK as a reliable partner but have found the government’s recent actions to fall well short of your rhetoric.”