Millions of Brazilians were left unable to access digital vaccine certificates after the country's health ministry was hit by a massive cyber attack on Friday.

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Hackers took several systems offline and posted a message on the health ministry's website saying that vaccine data had been copied and deleted.

"Contact us if you want the data back," the message read, including what appeared to be the hackers' e-mail and Telegram contact information.

The health ministry said it was working to restore the systems and minister Marcelo Queiroga told reporters that no data will be lost, claiming that the ministry still holds all the information that was stolen by the hackers.

But the attack has seen the government delay the implantation of new Covid-19 regulations by a week. The rules are set to include a rule requiring unvaccinated travellers to quarantine upon arrival in the country.

Brazil is widely viewed as being wildly under-prepared for a cyber attack and earlier this year a huge data leak saw tax numbers, facial images and addresses belonging to 223 million people posted for sale on the dark web.