Transport chiefs have issued a safety warning over charity balloons as they prepare to cope with an influx of passengers on Great North Run weekend.
Metro bosses have urged anyone travelling with a balloon to ensure it is kept on a secure, short string and they keep "tight hold" at all times.
No major incidents have been reported involving balloons in previous years. However, they can prove a danger on transport systems such as the Metro, which use overhead electric cables.
In 2015, a child's balloon caused a dramatic explosion which knocked out the metro system in Madrid, Spain, after it floated through the station and hit an overhead line.
There have been a number of incidents of balloons impacting on rail services in the UK in recent years, and the floating novelties are banned in Amsterdam's Schipol Station after a number of incidents of balloons hitting lines in the tunnel with smoke and smouldering particles causing train delays and station evacuations.
Balloons can also pose a danger of electrocution.
A spokesman for Nexus said: "For safety, anyone intending to travel on Metro with a charity balloon, is also advised to ensure the string is secure, kept short and you have tight hold of it at all times."
The warning comes as thousands of runners, spectators, friends and family are expected to use the Metro system over what is one of the busiest weekends in the North East.
Extra Metro services will be in operation for this year’s Great North Run on Sunday September 10 - and runners and spectators are being advised to allow extra time for their journeys and to buy tickets in advance.