The military police, officials and retired immigration staff have been put on standby with the Public and Commercial Services union scheduling a 24-hour walkout tomorrow due to a row over jobs and pay.
Members of the union include immigration officials posted at Heathrow and other airports.
With the Olympics scheduled to begin on Friday, tomorrow is forecast to be one the busiest days in Heathrow's history.
The extra staff is on standby to prevent a chaos and for smooth fuctioning of work on what promises to be a very busy day in London.