Brazilian environmental officials and federal prosecutors say they sent a warning that farmers and land-grabbers in the Amazon were planning a day of coordinated fires on 10 August to send a message to far-right president Jair Bolsonaro, but authorities failed to act.

Wildfires and burning deforested land are common during the Amazon’s dry seasons but peaked this month to more than 26,000 – the highest August figure since 2010. The environmental disaster has taken on international dimensions and overshadowed the G7 meeting in Biarritz.

Federal prosecutors in the Amazon state of Pará have now launched an investigation after revealing that they warned Brazil’s environment agency Ibama that a “fire day” demonstration was being planned around the town of Novo Progresso.

The prosecutors noted that the local Folha do Progresso news site had reported on plans for the fire day 5 August. “We need to show the president that we want to work” one local farmer told the website, adding that the only way to clear land for pasture was by felling trees and burning them.

But Ibama did not reply to the warnings until two days after the protests began, when it said that its operations had been hampered because police support had been withdrawn, putting their teams at risk in a region where they already face threats.

“It was a considerable failure,” prosecutor Paulo Moreira Oliveira told the Guardian. “There should have been immediate action to confront the risk of these fires.”

A separate investigation is examining the rise in deforestation on public land in Pará and whether public bodies and authorities could be responsible. Moreira Oliveira said that Ibama has imposed fewer fines for deforestation in Pará even though forest clearance has increased since Bolsonaro took office.

Two environment officials with experience in the region told the Guardian they also knew about the fire day beforehand. One official at the Chico Mendes Institute in Pará– which, like Ibama, is part of the ministry of the environment – said officials had asked bosses in Brasília for help, but requests for reinforcements were ignored.

“I know support was requested for an emergency plan, but it was not answered,” the official said. Both spoke anonymously because the government has banned environment officials from talking to media.

Brazil’s prosecutor-general Raquel Dodge said on Monday that there was a “suspicion of orchestrated action,” the G1 news site reported.

Bolsonaro has repeatedly attacked Ibama for running a “fines industry” and vowed to open up the Amazon for development.

He first blamed the fires on NGOs but provided no evidence, then conceded that farmers were also setting land on fire to increase productive areas and called on them to stop.

On Sunday, Brazil’s justice minister Sergio Moro tweeted that Bolsonaro had asked for a “rigorous investigation” and said federal police had been activated, after environment minister Ricardo Salles tweeted an article from the Globo Rural site about the “fire day”.

The site reported that about 70 rural farmers, land grabbers and businessmen Novo Progresso and Altamira coordinated ‘fire day’ on the margins of the BR-163, a highway which leads through heavily deforested areas.

“These people are Bolsonaro’s electoral base,” said another environment official who has worked in the area. “The last thing they want to know about is protecting the Amazon.”

On Monday, the leader of a farmers’ union in Novo Progresso denied that there had been a plan for the fire day. “We have no knowledge of this. If there was anything like that, it was an isolated act,” Agamenon Menezes told the Agência Brasil news agency.

The Brazilian environment ministry did not respond to a request for comment.