LONDON — In a surprising courtroom reversal, Husnain Rashid, a self-described jihadi accused of encouraging a terrorist attack on the school of 4-year-old Prince George, the third in line to the British throne, changed his plea Thursday and admitted a series of terrorism offenses that could bring life imprisonment.

The case fused two conflicting themes in the nation’s public life: its enduring preoccupation with the royal family and the struggle to avert terrorism a year after a suicide bombing in Manchester followed by attacks on bridges.

According to court testimony, Rashid used an online messaging service in October to publish a photograph of George’s school in London, with silhouettes of jihadis superimposed on it. He also gave details of the school’s address and a warning: “Even the royal family will not be left alone. School starts early.’’

While that threat inspired many headlines, prosecutors said the royal family was only one of many targets suggested by Rashid, 32, including soccer stadiums, British army bases, Jewish institutions, polling booths, the Wimbledon tennis tournament, and concert sites. He had also called for ice cream and fruit to be poisoned.

“His proposals were indiscriminate and made no distinctions between adult and child, between members of fighting forces and civilians,’’ prosecutor Annabel Darlow told the court.

Rashid was arrested in November at his home in northwest England and his trial opened a week ago when he pleaded not guilty to all charges. On Thursday, though, he changed his plea to guilty and will now be sentenced in June.

NEW YORK TIMES