A former policeman has admitted illegally possessing 650 wild bird eggs collected while he was still a serving officer, the RSPB has said.

A search of Michael Upson's house in Sotherton, Suffolk, revealed 650 wild bird eggs, including protected species such as woodlark, Cetti's warbler and marsh harrier, the charity said.

Upson, aged 52, pleaded guilty to possessing the eggs at Norwich Magistrates' Court following a joint investigation by his old force Suffolk Police, Norfolk Police and the RSPB.

Notebooks found at his house documented Upson's egg collecting trips with others around the UK.

A selection of Michael Upson's marsh harrier eggs

On one occasion they showed he took kittiwake eggs from Lowestoft Pier while on duty as acting sergeant on three different night shifts.

The notebooks also detailed visits to the Western Isles to steal golden eagle eggs, south Devon to take Cetti's warbler eggs, North Wales to steal chough eggs, and the New Forest to take hawfinch eggs, the charity said.

In a diary entry for April 1997 the police officer describes in detail finding the Cetti's warbler nest, then a scarce species, and placing the fresh eggs in a margarine tub.

Many of the eggs were stored in a suitcase the loft

He wrote: "I then started on the long journey home absolutely thrilled and delighted with the clutch and pleased I had learned a little bit more about the nesting habits of a bird completely new to me."

Upson claimed to have stopped egg collecting. The evidence found indicates he was active between at least 1991 and 2001, the charity said.

Mark Thomas, RSPB investigations officer, said: "Evidence from the diaries indicates that Upson stole over 900 wild bird eggs in a eight-year period. Not all of these eggs were recovered.

"That a police officer should knowingly break the law in pursuit of this obsession is shocking, and we welcome his conviction."

The egg collection was found in an old suitcase in Upson's loft, along with hundreds of egg data cards, which he had faked to suggest the collection was old.

The notebooks found in a plastic container hidden in the water tank in the loft gave the accurate details of when the eggs were taken, in full written accounts, according to the RSPB.

Upson is due to be sentenced at Norwich Magistrates' Court on Thursday.