Planning permission has been given to build safety structures at the site

A report in the Sunday Express cites a company source claiming the aircraft came within 100ft of the cooling tower at the Cumbrian plant in December.

But British Nuclear Fuels (BNFL) said it rejected the claims calling them "without foundation".

BNFL is set to erect new safety structures at Sellafield.

Alan Hughes, a BNFL spokesman, told BBC Newsonline: "The alleged incident simply did not happen."

He said if a fighter jet was to come within 100ft of Sellafield, people living nearby would have known about it and called police.

Enhance security

He said: "If this had happened there would be no way that we could have kept it secret even if we wanted to."

RAF regional community relations officer for Cumbria, Tony Parrini, also said they had no record of such an incident.

He said: "As far as I have been able to determine, there is nothing to substantiate the story.

"If they would like to submit it with dates and times it would be fully investigated by the Ministry of Defence."

New safety work is being carried out at Sellafield but no details are being released for what the company says are security reasons.