A £12m decorative ceiling for a United Nations building in Geneva has come under fire after it was disclosed that it was partly paid for by Spain’s overseas aid budget.

The decision to hire Miquel Barcelo, 51, one of the world’s most highly paid abstract artists, to redesign a 14,000 sq ft dome has prompted furious protests from campaigners who believe it is an extravagant misuse of development funds.

Barcelo has said his design aims to create a “grotto”, with stalactites reflecting “infinity and the multiplicity of view-points”. He has built a honeycomb of aluminium from which to hang resin stalactites up to 3ft long. They are coloured with paint containing pigments from all over the world.

The work, at the headquarters of the UN Human Rights Council, will be unveiled this week by Ban Ki-moon, the UN secretary-general, King Juan Carlos and Queen Sofia of Spain and Jose Luis Zapatero, the Spanish prime minister.

Zapatero’s socialist government has admitted contributing £6m to the cost of the ceiling, hailed by its largely Spanish sponsors as one of the UN’s most important works of art. The money includes £420,000 from Spain’s African aid budget. “This is an immoral use of these funds and we will establish whether it is illegal,” said Gonzalo Robles, an MP from the opposition Popular party.

“How many vaccines, how many wells, how many thousands of children in different countries could have been helped with this money?”

The lavishness of the project has also caused disquiet among UN staff. “It’s shocking that development aid was used for this,” said an employee.

Javier Garrigues, Spain’s ambassador to the UN, said the location justified the spending of aid money. “Funding the human rights headquarters clearly falls into the category of development aid,” he said.

Additional reporting: Sara Hashash

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