Posts claiming that the flags outside parliament have been changed from the EU flag to the Union Jack for the opening of parliament by the Queen have been shared over a thousand times on Facebook.
This is inaccurate. There were no flags in the square in the weeks before these were put up —EU or otherwise. These flags were raised sometime between Friday 11 and Saturday 12 October.
Additionally, the EU flag is not officially in Westminster or Whitehall, to begin with. The EU flag has previously been flown in Parliament Square alongside the flags of other European countries (not just of the EU) to mark Europe Day.
The flags are also not a permanent feature in the square—a scroll through Google street view shows a variety of flags being erected and removed from the square. This has been due to certain national and international celebrations.
The Union Jack is traditionally flown around London during the state opening of parliament. You can see the flag being flown outside parliament during the 2016 ceremony.