In the history note at the conclusion of this (excellent) book, Sansom correctly says that after Henry VIII's death, Catherine Parr married Sir Thomas Seymour and Princess (later Queen) Elizabeth went to live with the couple.
He adds, almost as an aside, that when Catherine fell pregnant, Seymour sexually abused the 14 year old Princess.
It comes from a written report by Kat Ashley, the Princess's governess. I don't think that anyone disputes what she said, but of course no one can now prove the truth of it. http://www.elizabethfiles.com/catherine-parr-and-thomas-seymour-part-two/3660/
it's sometimes suggested as the reason Elizabeth kept men at arm's length during her reign (though doubtless she also had political reasons for doing so)
At the time it happened, she was fairly powerless and, at times, even in danger of death. It might have been piquant to Tom Seymour to demonstrate his power over the daughter of a king in such a way.
Edward Seymour, Lord Protector, was executed in 1552, after the Earl of Warwick, later Duke of Northumberland, took over affairs of state and Seymour was convicted of felony in plotting to bring down Warwick's regime. Northumberland did not last long; after the death of Edward VI he put his daughter in law Jane Grey on the throne. When the Privy Council had a change of heart and proclaimed Mary Tudor as queen, he lost his head.
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