Portugal’s prime minister, Antonio Costa, resigned on Tuesday amid an investigation into alleged irregularities committed by his majority Socialist administration in handling lithium mining and hydrogen projects in the country.

Mr Costa announced his decision in a televised statement after meeting President Marcelo Rebelo de Sousa. He said his conscience was clear, but he would not stand as candidate for the premiership again.

Prosecutors earlier on Tuesday named Joao Galamba, the infrastructure minister, as a formal suspect and detained Mr Costa’s chief of staff. The opposition has demanded that the government step down with immediate effect.

Mr Costa said he was “fully available to cooperate” with the justice system after prosecutors said he was also the target of an investigation.

“The dignity of the functions of prime minister is not compatible with any suspicion about his integrity, his good conduct and even less with the suspicion of the practice of any criminal act,” Mr Costa told reporters.

Mr Costa is among several people being investigated in the case of two lithium and hydrogen businesses, according to a statement from the Attorney General’s Office. Suspects have named the prime minister as someone who could intervene and unblock the procurement of licences, the statement said.

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