Flynn and Gethins, who won Aberdeen South and Arbroath and Broughty Ferry respectively for the SNP at the 2024 general election, had to resign their seats to take up a place at Holyrood.
The SNP’s rivals are hoping to overturn Flynn’s majority of almost 4,000 votes in Aberdeen South, and Gethins’ majority of about 900 votes in Arbroath and Broughty Ferry.
Meanwhile, in north west England, Labour’s Mayor of Greater Manchester Andy Burnham is widely anticipated to launch a bid to oust Starmer as prime minister if he wins the Makerfield by-election.
For each of the by-elections, voters must have photo ID, external to cast their ballot at polling stations.
Unlike the recent Holyrood election, counting will take place overnight with the results expected in the early hours of Friday.