The ban led to extraordinary scenes at the House of Commons on Friday when Mr Nelson, 42, was escorted from the precincts by a police sergeant.
Police could not tell the mystified Mr Nelson why he was barred, but it emerged that the Serjeant at Arms, Peter Jennings, had written to the People, saying that the political editor's Commons pass had been withdrawn for 20 sitting days.
Unfortunately, the letter had been sent to an address vacated by the People three years ago.
The Serjeant at Arms called Mr Nelson to book for revealing that booby- trapped letters containing tiny sharpened metal blades had been delivered to Sir David Steel MP and the Labour peer Lord Donoughue. They are thought to be the work of animal rights terrorists.
Parliament is deeply sensitive about security matters, fearing "copycat" attacks. Mr Nelson insists that he cleared his copy with the security department.
Mr Nelson was warned by a previous Serjeant at Arms that his pass was "hanging by a thread" after reporting the discovery of a used condom in the Commons chamber. No action was taken over a later story of "lesbian sex romps".
Mr Nelson is to appeal against the ban on Tuesday when the House reassembles. "Basically, I have been sentenced without a trial," he said last night. "I see my role as to write about news."Reuse content