Prince Harry, Baroness Lawrence and Sir Elton John are among those bringing legal action against Associated Newspapers Limited (ANL), which publishes the Daily Mail and The Mail on Sunday, over allegations of unlawful information gathering.
The publisher strongly denies the claims and is defending the case brought by the group which also includes Ms Frost, Liz Hurley, David Furnish and Sir Simon Hughes.
The group's barrister, David Sherborne, previously said in written submissions that part of Ms Frost's claim related to an unpublished article about an ectopic pregnancy in 2003.
The barrister continued that Ms Frost did not tell her mother or sisters about the pregnancy and subsequent termination, only telling her partner Jackson Scott, who was the father of the unborn child, and 'maybe also one of her very close friends'.
Mr Sherborne said that Katie Nicholl, former diary editor for The Mail on Sunday, had found out details about the pregnancy and the termination, which 'must be from unlawful information gathering'.
Giving evidence remotely on Monday, Ms Nicholl said the tip for the story came from a freelance journalist 'with a very good source', rather than via unlawful means.
Mr Sherborne suggested Ms Nicholl had used enquiry agents for the information, but the journalist replied: 'I never used them to blag medical information. I can be very clear that I did not blag medical information.'
The former diary editor of The Mail on Sunday did not 'blag' medical information from Sadie Frost (pictured in January), the High Court has heard
The barrister said Ms Nicholl asked Ms Frost 'an extraordinarily intrusive question' about whether she had been pregnant in the last 12 weeks, and suggested she needed the information to be confirmed by Ms Frost or a family member 'so you had a legitimate source for the story if anyone complained'.
However, Ms Nicholl said she was trying to 'second source' the story, adding: 'I was trying to stand up a story, she denied it and we did not run it... what I am clearly trying to do is put the story to her and give her a chance to respond.'
Ms Nicholl later told the court there were multiple reasons for the decision not to publish the story, including Ms Frost's denial 'and it being a deeply sensitive medical story', as well as the involvement of her lawyers.
The journalist is also named as an author of seven articles in Harry's claim against ANL, which the Duke of Sussex alleges came from unlawful information gathering.
But in her written evidence, Ms Nicholl said that she had 'an excellent, reliable network of contacts', and that she got many stories about Harry from friends.
She continued: 'They had an intimate knowledge of his social life both through spending time with Prince Harry and his group of friends and having access to his social media.
'These were acquaintances who knew him or knew people very close to the prince.'
One article in Harry's case related to him being chosen as godfather to the child of his former nanny Tiggy Legge-Bourke.
In her written evidence, Ms Nicholl said that while she was not '100 per cent sure' who she had spoken to for the story, she often spoke to the late Queen's close confidante and first cousin once removed, Lady Elizabeth Anson – a 'close friend' of Ms Legge-Bourke.
The trial before Mr Justice Nicklin is due to conclude later this month, with a judgment in writing due at a later date.
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