Image: Donald Trump speaks to the 80th session of the UN’s general assembly (UNGA) at UN headquarters in September in New York. Photograph: Alexi J Rosenfeld/Getty Images

The Trump administration’s long-anticipated decision this week to pull the US from the world’s most important climate treaty may have been illegal, some experts say.

“In my legal opinion, he does not have the authority,” Harold Hongju Koh, former head lawyer for the US state department, told the Guardian.

In a Wednesday presidential memorandum, the president said the US “shall withdraw” from the UN Framework Convention on Climate Change (UNFCCC), along with 65 other organizations, agencies and commissions that it deemed “contrary to the interests of the United States”. It marks the first time any country has ever moved to exit the agreement.

The UN climate body requires one year’s notice for withdrawal, so the United States will not cease being a party for a year. Trump’s memo did not specify whether or not his administration will submit a formal notice of termination to the UN.

Reached for comment, a state department spokesperson noted that Trump’s memo said the agency will take all necessary “steps to effectuate the withdrawal of the United States from the organizations as soon as possible”. The person also referred the Guardian to secretary of state Marco Rubio’s Wednesday statement, where he said the Trump administration is exiting institutions and treaties which it believes to be “redundant in their scope, mismanaged, unnecessary, wasteful, poorly run, captured by the interests of actors advancing their own agendas contrary to our own, or a threat to our nation’s sovereignty, freedoms, and general prosperity”.

Because the country entered the UNFCCC with the consultation and approval of the Senate in 1992, “there is an open question” about whether or not the president can unilaterally exit the agreement, Michael Gerrard, a climate law expert at Columbia University, wrote in an email.

The UNFCCC and the Paris climate agreement – the landmark 2015 climate pact underpinned by the UNFCCC, from which Trump pulled the US last January – both say that parties may withdraw with one year’s written notice. But unlike the UNFCCC, the Paris agreement was never ratified by the US Senate, Gerrard noted.