“There’s no control over the decorative rubbish and who sells what,” added Ms. Gettleson, who has been a vigorous campaigner for the preservation of antiques at the market through the “Save the Portobello Road Market” Facebook page, even though her showroom has largely been given over to the London Tree House’s Paddington Bear-themed gift shop. “There’s still a core of experienced dealers, and astonishing things still sell in this market,” she said.
Nonetheless, more of Portobello’s hundreds of indoor and outdoor retail units have been given over to “new goods” such as Scottish knitwear, amber jewelry and London-themed souvenirs.
The 1996-1997 Guide to the Antique Shops of Britain listed more 700 active dealers in the Portobello Road arcades. Mark Atkinson, the American-born markets development officer for the Royal Borough of Kensington and Chelsea, which includes Portobello Road, put the current figure at 300 to 400.
“Antiques make Portobello distinct from other destination street markets in London, like Brixton, Camden and Brick Lane,” said Mr. Atkinson, whose department provides 54 outdoor “pitches,” or spaces, for Portobello antiques traders on a Saturday.
In 2009, the Portobello Group, which is owned by the British real estate investor Warren Todd, replaced 79 dealers in Lipka’s Antiques Gallery with a branch of the AllSaints fashion chain. The independently owned arcades Geoffrey Van (closed 2010), Good Fairy (2011) and W. Jones (2016) have been further casualties.
Yet the tourists come in droves. Portobello Road, like Venice, faces the challenge of being a place that millions of visitors want to “experience.” According to research commissioned last year by the Borough of Kensington and Chelsea, about 5.5 million people visited the “core areas” of Portobello Road in 2016. About 60 percent of the visitors on Saturdays came from outside Britain. Significantly, the data, supplied by Vector Research, showed that 47 percent were “browsers” — people who did not make significant purchases — with just 15 percent driven by an interest in antiques.
“The Lonely Planet guide told us it was a typical place to visit in London,” said Fred Eral, 43, a chemical worker from Arles, France, who was visiting Portobello with friends for the first time.
By noon, loneliness is hard to come by as the selfie-taking crowd flows northward up Portobello Road toward the stalls where Hugh Grant’s character bought vegetables in the movie “Notting Hill.”
“There’s an atmosphere here,” said Lyndon Grady, 54, a former comedian, who is now a dealer in rare and secondhand books. Ms. Grady said she was more than happy to cater to the crowds of foreign tourists pouring past her roadside stall, which displayed hardback editions of Jane Austen, “Winnie-the-Pooh” and “Just William” stories. “I sell a lot to Americans and Chinese,” she said. “The Chinese love poetry.”
Ms. Grady’s space cost her £55 for the day. By 11 a.m. on a recent Saturday, she said she had already covered that expense with £70 of sales.
“Today is very dead. The people just look and go,” said Sahin Sheikh, cursing the cold drizzle. His sidewalk stall at the southern end of the market specializes in wooden iPhone covers, priced at £10 each. Nearby, another street trader was trying to sell a Picasso-style ceramic plate.
“There’s certainly a segment that’s basically a tourist trap,” said Mr. Atkinson, the markets development officer.
Despite these issues, he said he remained optimistic about the market’s future.
“Portobello may not be the same as before, and it may not have the same scale. But the place is important,” added Mr. Atkinson, who puts his faith in the so-called analog renaissance among young millennials. He said that visits to Portobello by the pop star Taylor Swift and by the N.F.L. player Brandon Marshall — both widely flagged on social media — were “emblematic of this trend.”
Mr. Todd of the Portobello Group did not respond to calls and emails requesting comment. His son, Ryan Todd, principal of the Portobello Group, said in an email that “‘new goods are of course a problem.” He added: “We do our best to ensure we opt for quality where possible, but quality demand has dwindled of late and with increased costs to consider, we operate a business and occasionally have to make difficult decisions.” He added that the relentless rise in London’s business taxes, soon to increase in the Portobello area by as much as 50 percent compared with 2010, was a major issue.
“Portobello is a complex organism,” Mr. Todd said. “We are committed to doing all we can to retain that character where possible.”
However, traders said that some antiques dealers were giving up because the market was overrun by tourists and by counterfeit wares.
“A lot of dealers are annoyed they’re surrounded by fakes. But where else is there to go in London that’s still affordable?” said Matthew Holder, who, at 25, is one of the few younger dealers with a booth in the antiques-filled basement area of the Admiral Vernon arcade, owned by the Portobello Group. Mr. Holder specializes in medieval and Renaissance objects. He pays £500 a month for his four trading Saturdays.
“It is getting harder,” said Mr. Holder. “There’s less trading between dealers, and the big dealers don’t really come to the market anymore.” But he maintains that he would never give up Portobello. “I don’t just want to do Instagram and fairs. I like to get here at 5 a.m. on a Saturday morning,” he said. “I like to deal.”