Portugal's government says there is "no truth" to the document, which has been shared widely on social media.
A fake document circulating on WhatsApp has claimed to show Portugal's latest plans for lifting its COVID-19 lockdown.
The government of Prime Minister António Costa has said the image is "false" and has filed a complaint with the public prosecutor. Portugal introduced a national lockdown on January 15, which remains in place.
The misleading file began spreading on social networks on Thursday, including via the popular WhatsApp messaging platform.
An image showed a table of dates with some pandemic restrictions supposedly lifted by May, alongside the official logo of the Portuguese government.
But authorities say the file was a version of their 2020 deconfinement plan, which had been "misappropriated" and tampered with by third parties.
In a statement on Thursday, the Prime Minister's office said that "no credibility" should be given to the false information.
"This document has no truth, is not of the Government's authorship, nor is it based on any preparatory work," the office said.
"Due to the misinformation and false expectations that such a document may generate, with the inherent risk to public health, this forgery will be reported to the Public Ministry."
The false document was an "abusive adulteration" of the deconfinement plan that the Portuguese government had published last April, the office added.
An investigation by Euronews found that the false deconfinement plan had been forwarded by a significant number of WhatsApp users and contained noticeable formatting irregularities.
At the start of the pandemic in March 2020, Portuguese health authorities warned citizens about a number of voice messages from fake health professionals circulating in WhatsApp groups.
"The publication and sharing of false information is one of the greatest risks to public health when it comes to emerging problems such as COVID-19," said Graça Freitas, Portugal's director-general of health.
'This is not yet the time for deconfinement'
According to the European Centre for Disease Control, Portugal's 14-day case rate for COVID-19 per 100,000 inhabitants stands at 287.
A surge in confirmed cases at the start of 2021 made the country one of the world's worst-affected by the size of its population.
This week, the country's Parliament approved a request by President Marcelo Rebelo de Sousa to extend the current state of emergency, due to end on March 16, by another two weeks.
Although positive cases have receded since a lockdown was introduced on January 15, authorities are wary of lifting restrictions early.
Prime Minister Costa has said that plans to lift the restrictions, including a strategy for testing and vaccinations, will be announced in due course.
"The Government is preparing the future steps of deconfinement," the office stated, "however, the Government considers that it is inopportune to proceed at this stage to any presentation or public discussion on the subject."
"This is not yet the time for deconfinement. On the contrary, as stated in the President's draft decree, experts do not recommend reducing or suspending contact-restriction measures in this context."
Portuguese Health Minister Marta Temido has also told lawmakers that any plans to ease restrictions must be based on recommendations by health experts.
"There is still a long way to go,'' Temido said.