To some he is an urban icon, a street artist dedicated to bombing his tag on more, and riskier, places than any other in the UK.

But Daniel Halpin – or Tox, "king of taggers" and scourge of London Underground's cleaning force – faces the possibility of prison walls as his only canvas after a jury decided his art was vandalism and convicted him of criminal damage.

The 26-year-old, from Camden, north London, whose masked image and story of anarchism has featured on television documentaries and in magazines, was found guilty of a string of graffiti attacks across England after prosecutor Hugo Lodge told a jury: "He is no Banksy. He doesn't have the artistic skills, so he has to get his tag up as much as possible."

As he was remanded in custody for sentencing, his artistic merit was further questioned by the reformed guerilla graffiti artist turned establishment darling Ben "Eine" Flynn, whose work was presented to the US president, Barack Obama, by the prime minister, David Cameron, last year.

"His statement is Tox, Tox, Tox, Tox, over and over again," said Flynn after the trial at Blackfriars crown court, in which he gave evidence as an expert witness. In his opinion, the Tox "tags" or signatures, and "dubs" (the larger, often bubble lettering) were "incredibly basic" and lacking "skill, flair or unique style".

Halpin, found guilty of seven counts of criminal damage, was convicted alongside Daniel "CK1" Fenlon, 25, from Bristol, who was found guilty of one count. Goldsmith College student Gordon McDermott, 24, who the prosecution alleged was known as Cut and sometimes Miz, was acquitted.

Nicholas "Host" Rowley, a former student at Edinburgh College of Art, and Riga "Rigz" Paizis, who worked in a graffiti shop, both admitted six counts of criminal damage and await sentencing along with Halpin and Fenlon.

The five were arrested as part of British Transport Police's (BTP) Operation Misfit, which claimed to have identified their tags in Paris, Lille, London, Glasgow, Bristol, Leicester, Market Harborough, Kettering, Chippenham, and even on a funeral home in Bath.

Halpin – whose tag is simply Tox followed by the last two digits of the year – had claimed he was the victim of imitators. He said he had "retired" in 2005 after a career defacing buses, trains, bridges and walls earned him a string of asbos, which he largely ignored, and community service orders.

Cashing in on his notoriety, he is said to have made £9,000 in two hours by selling pictures with his Tox tag. Reports in 2009 that he was selling 100 canvasses bearing his notorious mark, at £75 each, precipitated heated debate. Purists condemned him for "selling out", while legal experts mused over whether a loophole made him impervious to the Proceeds of Crime Act.

But far from retiring, the Blackfriars jury was told, Halpin – acclaimed "king of taggers" by graffiti magazine Crack and Shine – had remained active and been caught on CCTV in Paris and London. The jury heard that what he lacked in talent he made up for in unrivalled willingness to scramble to hard-to-reach and risky spots.

"I don't know where you can't see a Tox tag – they are in places even I don't know how to access," one London Underground manager once admitted.

Debunking Halpin's defence of an army of imitators, Lodge told the jury: "Every time he talks about being Tox, his face lights up. He can't help but smile. He hasn't retired. He has turned professional. To maintain this, he has to keep getting his tag up. It's everywhere, and it's him."

Following Tuesday's verdict, judge Peter Clarke QC said of Halpin, who has spent 150 days in custody since his arrest: "The simple fact is the evidence effectively says he hasn't given up."

Flynn, 40, a married father of three living in Hastings, was called by the defence to offer his opinion on whether the tags could have been the work of impersonators, and said the Tox tags he had been shown were so basic that "pretty much anybody could quite easily duplicate it".

But that was the fate of graffiti art today, he said, with more secure train yards, fences, razor wire and increased security patrols allowing less time to be creative.

The appearance of Tox's tag in gilt-framed canvasses was "well funny", Flynn said, adding: "Art is worth what people are prepared to pay for it." People must have bought them as an investment, he added. "I can't imagine they bought them because they actually like them."

Detective Constable Will Livings, of the BTP Graffiti Unit, said: "Some people consider graffiti to be art but in reality it is nothing more than selfish vandalism that not only scars the railway environment but contributes to fear of crime and costs operators thousands of pounds in equipment downtime, as well as cleaning." BTP would "always seek to catch and prosecute those who commit such crimes", he said.

From graffitist to artist

Ben Flynn, in his time as hardcore graffitist Eine, clocked up between 15 and 20 arrests and five convictions for criminal damage before becoming a legitimate artist, with his TWENTYFIRSTCENTURYCITY work adorning the walls of the White House. After Halpin's conviction, he said: "We would spend days drawing what we were going to paint that weekend. When I wrote graffiti, I knew I would have maybe an hour or an hour and a half to paint.

"Now, there is less time to do something nice. They have only five or 10 minutes, so they are not going to spend their time in their bedrooms developing intricate graffiti. So graffiti has evolved into something that is less easy on the eye."