The Monaco, Azerbaijan and Canadian Grands Prix are all at risk of cancellation because of the ongoing impact of COVID-19, according to Formule 1.
The Dutch news outlet report that the three street circuit races could be at risk due to the organisers not wanting to front the cost for an event that may not take place.
The planned season-opening Australian Grand Prix, another street circuit, has already been postponed until November, while the Chinese Grand Prix was also removed from the calendar.
The report suggests that an announcement will be made within the next month, with the Algarve Circuit in Portugal a candidate to come onto the calendar and further rejigging likely to happen.
The COVID-19 pandemic wreaked havoc on the 2020 calendar, with the season postponed until July and an abridged 17-race season drawn up.
Despite the lessons of last year, Formula 1 and the FIA pushed ahead with an ambitious 23-race calendar for the 2021 season and that already looks to be backfiring.
While a vaccine for COVID-19 is being rolled out worldwide, people are being offered the jab at different rates from country to country and numerous travel laws remain in place.