Arsenal have launched an investigation after urban climbers breached security to film themselves inside and on top of the Emirates Stadium.
A climber, who calls himself DyingLlama, and his friends scaled the stadium's walls and posted video images on Youtube showing them wandering around the stadium.
The youngster then claims on the video that he and six other friends were able to breach security at West Ham's home ground, the London Stadium, where they also wandered pitchside, in the stands and on the roof.
It is the second time security at the London Stadium has been breached in this way by urban climbers following a similar stunt carried out by a group known as Night Scape, who posted footage in October.
The latest incident at the Emirates Stadium is understood to have taken place nine days ago and has prompted the Premier League club to re-evaluate security measures at its north London ground.
An Arsenal spokesman said: "Fan safety is a top priority and we take security at Emirates Stadium extremely seriously.
"We have launched an immediate investigation into this incident. We work closely with police and anti-terrorism experts and will be reviewing our procedures."
The Emirates Stadium has 24-hour security staff and is monitored by CCTV, but its "robust security system" appeared to be easily side-stepped by DyingLlama and his friends.
The video footage shows them climbing a stairwell and walking through an unlocked door on to the terraces and then pitchside.
The climbers also hopped on to the stadium roof and perched directly above the iconic stadium clock in the Clock End.
Similar footage of the group breaching security at the London Stadium shows them looking down from the roof and walking pitchside.