An aircraft flown into the Sellafield nuclear plant in Cumbria could cause "at worst, several million fatalities", according to a leaked report.
Planning permission has been given to build safety structures at the site
New Scientist magazine, says it has seen a copy of a report by the Parliamentary Office of Science and Technology, which looked at risks of a terror attack on a nuclear facility.
The report follows a number of aircraft "near miss" incidents in recent years.
The company which operates Sellafield said such a scenario was implausible.
Over the last five years, the operators of 19 nuclear sites have lodged more than 100 complaints about both military and civilian aircraft flying too close, it is claimed.
New Scientist said it had been leaked the parliamentary report, which suggests that a 9/11-style terrorist attack on a nuclear installation could claim millions of lives.
Since the 2001 attacks in New York and Washington DC, the no-fly zone around nuclear sites in the UK has been doubled.
Aircraft are now banned from flying within a two nautical mile (3.7 kilometre) radius of a nuclear facility, and must also stay above a certain height which varies according to the location.
But these restrictions have been flouted on numerous occasions, claims New Scientist.
Declassified reports from the Ministry of Defence (MoD) revealed 56 breaches by military aircraft between 2000 and 2003.
Four complaints came from the MoD's own nuclear weapons sites at Aldermaston and Burghfield in Berkshire, and Faslane near Glasgow.
There had also been 71 complaints of civilian aircraft straying into no-fly zones since the beginning of 1999, said the magazine.
The Civil Aviation Authority (CAA) had sufficient evidence to launch formal investigations in just 12 cases, including two at the Sellafield plant.
In March the operators of Sellafield denied claims that an RAF jet came within a second of crashing into it.
New safety structures are due to be erected at Sellafield.