The vast majority of false information shared on WhatsApp in Brazil during the presidential election favoured the far-right winner, Jair Bolsonaro, a Guardian analysis of data suggests.

The analysis sheds light on the spread of misinformation on the Facebook-owned app, with fears it could be poisoning political debate in one of the largest democracies in the world.

In a sample of 11,957 viral messages shared across 296 group chats on the instant-messaging platform in the campaign period, approximately 42% of rightwing items contained information found to be false by factcheckers. Less than 3% of the leftwing messages analysed in the study contained externally verified falsehoods.

The figures suggest the spread of fake news was highly asymmetrical, accounting for much of the content being spread by and to Bolsonaro supporters on WhatsApp.

The instant messaging app is very popular in Brazil, with more than 120 million users in a population of about 210 million people.

Much of the fake news shared on WhatsApp reflected the far-right values promoted by Bolsonaro’s team during the two-month campaign. Almost all the major rightwing fake news stories fell into four categories, the analysis reveals:

Up to 48% of the rightwing items containing externally verified falsehoods mentioned a fictional plot to fraudulently manipulate the electronic ballot system, echoing conspiracy theories promoted by Bolsonaro’s team and casting suspicion on the democratic process.

Another 19% of the messages promoted misleading information about a stabbing attack against Bolsonaro in early September 2018. The stabbing, which forced the far-right leader to spend most of the remaining weeks of the campaign in hospital, was a turning point in the election.

Sixteen per cent of right-leaning false content tried to dismiss the political system and mainstream media as corrupt, reflecting key elements of Bolsonaro’s anti-establishment rhetoric.

A further 14% of the viral falsehoods targeted leftwing politicians and activists, often using homophobic tropes and anti-feminist slurs.

Content exchanged on WhatsApp is protected with end-to-end encryption, so the study collected a sample of messages from WhatsApp Monitor, a database of viral content shared in hundreds of groups focused on political debate. Due to these sampling limitations, the research cannot be generalised to explain the behaviour of all WhatsApp users in Brazil.

WhatsApp executives recently acknowledged Brazilian accounts were the target of massive spamming operations by digital marketing agencies before the election, in a breach of the app’s terms and conditions.

Inquiries in Brazil’s congress and superior electoral court are examining whether Bolsonaro’s team was involved in these manipulative efforts. The investigations could potentially lead to the annulment of the election result.

The Bolsonaro administration denies wrongdoing and has repeatedly dismissed media reports on the issue as “fake news”.

Facebook, which has been under increasing pressure to do more to tackle misinformation, announced an update of WhatsApp in January that reduced to the number of times users can forward a single message to five.

Recent studies suggested the upgrades succeeded in slowing the spread of false content, but failed to stop fake news reaching large audiences.

According to Caio Machado, a computational propaganda specialist at the Oxford Internet Institute, social media platforms are struggling to cope with “democracy-hacking strategies”.

He added: “Tech companies have an immense power in their hands, and no one knows whether or not they are actually capable of influencing election results.”

A WhatsApp spokeswoman pointed to changes such as labels showing forwarded messages, the limit on forwarding messages, banning spam accounts using machine learning and improved safety and privacy settings.

“WhatsApp has made significant product changes and worked with partners across civil society to help address the harmful consequences of misinformation,” she said.