(Adds detail, background, quotes) LONDON, May 5 (Reuters) - Belize’s bid for its fifth debt restructuring in 15 years is threatening to turn sour as holders of its so-called ‘Superbond’ urged it to agree to an IMF programme, while the government accused the creditors of cancelling a planned Zoom call. Serial-defaulter Belize is looking to restructure a $550 million Superbond that emerged from 2006-07 restructuring and now contributes to a 133% debt-to-GDP ratio that the International Monetary Fund deems unsustainable. The central American country has been hit hard by the impact of COVID-19, with a sharp fall in tourism revenue and a rise in support spending, but the new government elected in November has so far eschewed a formal IMF support plan. Superbond holders, U.S.-based funds GMO and Greylock, and London-based Aberdeen Standard Investments which recently rebranded as ABRDN, have formed a creditor committee and are signalling they may not restructure without IMF help. “The Committee respectfully reiterates that the implementation of Belize’s economic policies would be more durably addressed in the context of an IMF-supported programme,” the group said in a statement on Wednesday. They added the measures recommended by the Fund would help close a financing gap that has pushed Belize to seek to restructure its debt. Rebasing the benchmark year for economic growth to 2014 - an accounting adjustment which the IMF has estimated lifts 2020 GDP by 29% - could bring the debt ratio close to where Belize has said it would like it to be. The government in March announced its intention to restructure the bonds as part of a broader economic plan to restore fiscal sustainability. On Tuesday it said had been in close consultation with the IMF over the last several months and had developed a fiscal adjustment programme that Belize believed was broadly in line with the IMF’s recommendations. It said the bondholders had cancelled a video call with Minister of State Christopher Coye scheduled for Monday to discuss the proposed restructuring. “That meeting was cancelled after... the committee’s representatives indicated that the meeting was unlikely to be worthwhile unless Belize agreed in advance to accept a full IMF programme,” it said. It added the rebasing exercise had been stalled by COVID, while the committee had also asked Belize to consider “contesting the legal validity” of debt owed to Venezuela under the PetroCaribe programme, which saw Venezuela provide other Caribbean countries cheap oil. “Belize, as a responsible nation, has no intention of taking such an action,” the government said. (Reporting by Marc Jones and Tom Arnold Editing by Karin Strohecker, Peter Graff and Sonya Hepinstall) Our Standards: The Thomson Reuters Trust Principles.