Security breaches have been alleged before at Stansted
An investigation is underway at the Essex airport after the journalist and a photographer said they were able to get into the Boeing 737 and spend an hour with access to all areas of the plane.
Two security staff who were on duty at the hangar have been suspended while an investigation is carried out.
But Go has agreed immediate security improvements with maintenance firm FLS Aerospace.
Security office
The facility at Stansted has been closed, planes transferred to another maintenance site and an independent security expert is to be appointed to undertake a full security risk assessment.
The Mirror journalists drove into the staff car park of FLS Aerospace at the airport before walking into the hangar, it was reported.
They say they walked past a security office without being asked for identification and spent an hour in the hangar and in the plane, the newspaper reported.
The Go plane has been subjected to a fingertip search and other planes to pass through the maintenance facility may also be searched.
Heightened security
A spokesman for Go said: "We take this breach of security very seriously as safety is our most important priority.
"It is important that this issue has come to light and we are demanding action.
"We've met with FLS and the BAA and are working closely with the DTLR, Transec and the CAA to ensure that the cause of the problem is identified and immediately rectified.
The alert comes as there is increased focus on air security wake of the 11 September terrorist attacks.
Stansted's security was scrutinised last year after reports that a gun and a fake bomb were smuggled through a passenger checkpoint by an inspector.
Personnel suspended
A spokeswoman for FLS Aerospace said the alleged security breach on the 737 was being investigated.
"The security personnel who were on duty have been suspended without prejudice pending an investigation," she told PA News.
"We have implemented heightened security measures at the base, the aircraft has been triple-checked based on this alleged breach, and it would have undergone a normal full security check on completion of its maintenance."
Chris Yates, editor of the specialist publication Jane's Aviation Security, told the Mirror: "Given the events of September 11, it is appalling that non-airport staff should have been able not only to gain access to the sensitive airside of Stansted airport but also to board aircraft unchallenged."