The sentences given to Jordan Blackshaw, 20, from Cheshire, and Perry Sutcliffe-Keenan, 22, from Warrington, both in northwest England, were the toughest so far following four frenzied nights of violence in England last week.

Prosecutors said the men's online calls to riot last Tuesday caused "significant panic and revulsion" even though no one responded, because they occurred the same night as violence broke out in the nearby city of Manchester.

But justice campaigners and defence lawyers said the punishment was not in proportion to the crime, and urged the courts not to overreact as they struggle to process almost 1,200 people charged over the worst unrest for decades.

"The rush to send a message out is leading to some very bad sentences, which will be overturned on appeal," said Andrew Neilson, director of campaigns for the Howard League for Penal Reform.

He said it was "understandable" that the riots be treated as an "aggravating factor", but added: "In the Facebook case we're talking about four years' jail which would normally be associated with serious and violent offences."

The sentences compare with typical punishments of four years for robbing someone with a weapon.

During a visit to Cheshire, northwest England, Cameron backed the judge's decision, saying: "They decided in that court to send a tough sentence, send a tough message and I think it's very good that courts are able to do that."

He added: "What happened on our streets was absolutely appalling behaviour and to send a very clear message that it's wrong and won't be tolerated is what the criminal justice system should be doing."

Blackshaw and Sutcliffe-Keenan, who do not know each other, both pleaded guilty to intentionally encouraging another person to assist the commission of an indictable offence, and were sentenced at Chester Crown Court on Monday.

Blackshaw had SET up a Facebook page entitled "Smash Down Northwich Town", which encouraged people to gather "behind maccies" -- believed to be the McDonald's restaurant in Northwich town centre -- last Tuesday.

Only the police turned up, however, and he was promptly arrested.

Sutcliffe-Keenan's creation, "Let's Have a Riot in Latchford", called for people to riot the following day. The court heard he had been drinking, and when he awoke with a hangover the next morning, he removed the page and apologised.

However, prosecutor Martin McRobb said the pages created "significant panic and revulsion" amongst local people.

Although the two young men have received the longest sentences for riot-related offences so far, previous cases have indicated the courts are taking into consideration Cameron's earlier call for those responsible to be jailed.

Last week, a 23-year-old student with no previous convictions was jailed for six months for stealing a case of bottled water in Brixton, south London. The family of Nicholas Robinson gasped with disbelief when he was sentenced.

And at Manchester Crown Court on Monday, Judge Andrew Gilbart handed down tough sentences to three looters, saying that "outbursts" of criminal behaviour "must be met with sentences longer" than if they were committed in isolation.

Criminal defence barrister Paul Mendelle said some people must be imprisoned over the riots, but there were clear guidelines which must be followed.

"It appears the courts are being urged to ignore those guidelines and somehow to throw the rule book out of the window, and ramp up sentences. I find that unsettling and potentially unjust," he told BBC radio.

"It's often said that justice delayed is justice denied, but justice rushed can be justice denied as well."

A spokeswoman for the Judicial Office said however that there was "no truth" in reports that judges had been issued a directive to increase sentences.