Alan Passmore’s vile haul on two electronic devices showed babies and youngsters aged under 16, some in clear distress.
His sick collection was so large it was out of the bounds of police to log and file every image, magistrates in South Tyneside heard.
Passmore, of Pennycross Road, Pennywell, was charged with five offences, including making 322 indecent photographs or pseudo-photographs of a child at the most serious category A level, and one of extreme pornography.
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Passmore, 50, was also in possession of 205 images at category B and 1,726 images at least serious category C, prosecutor Niamh Reading said.
She added: “On March 17, 2020, a search warrant was executed, and devices seized from the defendant’s address.
“Both were found to contain indecent images. There were 300,000 images of children, many not categorised by the police.
“There are aggravating features of images and videos of children, and a number are moving images.
“The ages of the children are from zero to 15, and many of the images and videos show children in distress.”
Passmore pleaded guilty to three charges of making an indecent photograph or pseudo-photograph of a child and one of possession of a prohibited image of a child.
He also admitted possession of an extreme pornographic image portraying a sexual act with an animal.
The offences were committed between March 21, 2013, and March 18, 2020, in Sunderland.
Magistrates ordered an all-options pre-sentence report be compiled, meaning he could be jailed.
Passmore will be sentenced at Newcastle Crown Court on Tuesday, February 22.
He was made subject to the notification requirements of the sex offenders register.