Guinea Bissau President Umaro Sissoco Embaló has called for the amendment of the country's constitution, and set up a committee to come up with a proposal.
He gave the committee 90 days to come up with a proposal.
The country's current constitution dates back to 1984.
In a presidential decree issued Monday, President Embaló said the constitution contains "ambiguities".
"Before the contemporary challenges, the country has to have a system of government which adapts better to the sociocultural reality and also which contributes to guarantee institutions' stability," he said.
However, a human rights defender said the president's move is unconstitutional.
"The presidential decree which sets up a commission to amend the country's constitution is unconstitutional because it (amending the constitution) is a responsibility of MPs," Mr José Carlos da Fonseca, Guinean Human Rights league, head told DW Radio in an interview.
Mr Embaló took oath of office in February without the country's Supreme Court of Justice validating his victory.
In January, the National Electoral Commission (CNE) declared Mr Embaló winner of the December 29, 2019 re-run with 53.55 percent against his opponent Mr Domingos Simões Pereira's 46.45 percent.
Mr Pereira contested the election outcome before the Supreme Court. However, the court is yet to make a ruling and proceedings have been postponed over the Covid-19 coronavirus pandemic.
Last month, the Economic Community of West African States (Ecowas) acknowledged Mr Embaló as the winner of the Guinea Bissau re-run held on December 29 and urged him to form a government.
"After a deep analysis of the country's political situation, the regional heads of state decided to recognise Mr Umaro Sissoco Embaló victory of the December 29 presidential re-run", Ecowas said in a statement
The regional bloc also urged Guinea Bissau political actors to start working towards a constitutional amendment and a referendum.
Mr Pereira expressed regret towards the Ecowas decision.
"I want to make it clear that I will not negotiate Guinea Bissau state values. And all know that CNE is the only entity that can announce the electoral winner and not Ecowas," he said. "I am in deep sadness seeing an organisation like Ecowas abandoning its coup d'état zero tolerance principle".
The European Union has said the Ecowas decision puts an end to a long-term political impasse.
United Nations Secretary General António Guterres asked Guinea Bissau politicians to fulfil the Ecowas decision.
"Bissau Guinean political actors have to work in an inclusive and constructive way to implement Ecowas relevant decisions particularly in forming a new government and the appointment of a new prime minister," he said in a statement.
The Africa Union also congratulated Mr Embaló and urged Bissau Guinean politicians to adhere to Ecowas decisions and work for the country's stability.