Google is struggling to persuade people running "adult" blogs on its Blogger network that it will not necessarily delete them – after sending out a letter this week to thousands of blog owners threatening to delete their blogs from Monday.

The move has already led to high-profile defections, including Zoe Margolis, author of the female erotic memoir Girl With A One-Track Mind. She told the Guardian that she is exporting her entire blog to a separate domain in case Google wipes it without warning. She added, "Nine years I've been with them, but only four days' notice. Disgraceful."

The letter, apparently sent out earlier this week – just days ahead of the deletion – is part of a crackdown on pornographic advertising on its Blogger network, reckoned to have around 100m blogs. Normal estimates suggest that around 10% of blogs on large networks are pornographic.

In the letter, Google says it will "prohibit the monetisation of Adult content on Blogger. After June 30th 2013, we will be enforcing this policy and will remove blogs which are adult in nature and are displaying advertisements to adult websites."

But the ambiguous nature of the first sentence – which implies that any "adult" blog on Blogger that takes advertising will be wiped – has outraged and infuriated free speech campaigners.

Margolis, who runs the Girl With A One-Track Mind blog, tweeted: "I only have three days to export/self-host my entire blog? Fuck you, Google."

She told the Guardian that although she has not received a copy of the email from Google, she was moving her blog "just to be safe".

Violet Blue, a writer on sex topics, noted on Twitter: "All those things you liked about the early internet? Sexual expression/speech on public space blogs is a big one of those things." She called the move a "vague new anti-sex policy purge", adding: "It's wrong."

Google insisted to the Guardian that it does not intend to delete sites simply because they host pornographic material. Sources at the company pointed to the second sentence in the letter, and emphasised that it was intended to block sites that make money from outside sources.

Google is understood to be concerned at adverts pointing to outside websites for phone sex and affiliate links to larger pornography networks. It is thought adult sites that use non-adult adverts would not be targeted.

Google does not seem to be reacting to pressure on both sides of the Atlantic from campaigners against pornography, because it is allowing "adult" sites to remain. Instead, the move appears aimed at adult companies that are making money off the back of its Blogger network

"Adult" blogs on Blogger are identified as such by their owner, and would-be viewers have to check a box to confirm they want to view it before any content is shown.

Google declined to say how many people it has emailed with the warning, nor what percentage of blogs on Blogger might be affected. It bought the Blogger domain in 2003.

The company declined to explain the reasons for the move, or to say how many blogs – or what proportion of Blogger sites – would be affected.