A British Islamic State supporter has admitted to a string of terror charges including urging attacks on Prince George.

Husnain Rashid, 32, dramatically changed his plea from innocent to guilty two weeks into his trial at Woolwich Crown Court when he admitted Thursday to three counts of engaging in conduct in preparation of terrorist acts and one count of encouraging terrorism.

The trial, which had been scheduled to last six weeks, was abruptly ended after the plea reversal.

Rashid will be sentenced on June 28 for the offences spanning from October 2016 to April 2018.

He was found guilty of writing messages online encouraging militants to carry out attacks and posting a photo of the son of the Duke and Duchess of Cambridge next to a superimposed silhouette of a jihadi fighter.

He also posted the full address of George’s school with the caption “school starts early”.

“Even the royal family will not be left alone,” he wrote.

Prosecutor Annabel Darlow told a jury that Rashid encouraged attacks on a range of targets, including soccer stadiums, British Army bases, shopping centres, Jewish communities.

He also made plans to travel to Turkey and Syria to fight for IS, according to the Crown.

Husnain Rashid faces a lengthy jail sentence, according to the judge. Photo: AAP

Judge Andrew Lees told the defendant it was “inevitable” that he would face a “very lengthy prison sentence”.

“For the past week I have listened to the most disturbing allegations,” Judge Lees said.

“You have admitted these allegations of encouraging others to commit terrorist activities and publishing statements to encourage the killing of others.

“It is inevitable that you will receive a very lengthy prison sentence and there will be a consideration of a life prison sentence.”

“The question of your future dangerousness and the protection of the public is a matter that I will have to give very careful consideration.”

The prosecution also had argued that Rashid ran a “prolific” Telegram messaging channel named the Lone Mujahid.

IS is known to favour Telegram as a messaging system because its contents are encrypted and it keeps the user’s location hidden.

-with AAP