Two new embargoed biographies that promise to expose the truth about The Iron Lady are in publication, being delivered to printers under great secrecy.

The Authorized Biography Volume One: Not For Turning by Charles Moore is being published by Penguin Imprint Allen Lane at a cover price of £30, while a rival book Not for Turning, the life of Margaret Thatcher by her former speechwriter Robin Harris is being published by Transworld Publishers and costs £20.

The Allen Lane book was commissioned in 1997 on the understanding it would not be published until after Lady Thatcher’s death. Moore is currently working on the second volume.

Stuart Proffitt, a publishing director at Allen Lane said the biography "immediately supercedes all earlier books written about her," adding that Moore had revealed much which has never been made public before.

The book is being printed by St Ives' subsidiary Clays, which has an ongoing contract with the publisher.

Managing director Kate McFarlan said she received the print order yesterday and was expecting to start printing on Saturday. She was not able to reveal the initial print run.

"It’s a long book as it’s been planned for some time but we haven’t seen anything and we wouldn’t expect to see anything. The content is highly confidential," she said. "It’s a very special title but with our business we are used to having a set of procedures that click into place.

"We do a lot of books where the content is confidential and subject to an embargo."

Penguin also would not comment on the initial print run but the book, released on St George’s Day, 23 April, is expected to be a big seller and has already topped Amazon’s Political History of Politicians bestseller list on presale.

Clays is also benefitting from orders from other publishers to reprint other books about the former Prime Minister, including her own two-part autobiography published by Harper Collins.

Sensitive books have always meant great care has been taken with their contents but modern technology now allows for even greater security.

McFarlan said there was no need now for paper proofs and until the printer gets the order there is no need to know any details about the book at all. Both new biographies are set for publication next week.

Clays, which works with the majority of the trade publishing houses in the UK, prints and binds 160m books a year at one site in Suffolk.