FORMER PM Margaret Thatcher was threatened with a fine for failing to register for the poll tax, declassified documents reveal today.

The former prime minister’s introduction of the hated tax in 1989-1990 hastened the end of the Thatcher regime.

The tax, which took no account of the size of one’s property but was based on the number of occupants, was widely seen as simultaneously a sop to the rich and an attack on the most vulnerable and led to a mass campaign of non-payment and riots which shook London.

But even as the plans were being rolled out to persecute millions, Thatcher herself had to be warned she would be in breach of the law unless she completed her registration form on time, according to files published by the National Archives in Kew.

The embarrassing oversight, due in part to a bureaucratic wrangle between the Cabinet Office and Westminster City Council, was quickly rectified.

In early 1989, as the political storm was gathering strength, Westminster City Council, like other authorities around the country, began issuing registration forms in preparation for the launch of the tax in England and Wales the following year.

One form covering the various residencies in and around Downing Street, including No 10, which Thatcher and her husband Denis had nominated as their main home, was sent to the Treasury.

But the Cabinet Office complained that it was “most inappropriate” to issue a single form “asking a number of essentially personal questions” on behalf of individual occupants.

Tellingly this was not a concern extended to the rest of the country’s citizens, many of whom faced crippling bills as a result of the iniquitous “community charge.”

Individual forms were then despatched, but when they were also not returned, council registration officer David J Hopkins warned he would be forced to act.

In a letter dated May 22 1989, he said: “I wish to advise that you are required by law to supply the relevant information within 21 days of this request and failure to do so may lead to a penalty being imposed.”

The letter was addressed to the “Resident/Owner” at “Rooms First Floor, 10 Downing Street, London W1 9MN.”