A top Tory warned that a minority of the immigrants from Romania and Bulgaria could escalate existing problems, including pickpocketing and defecating on doorsteps.
Philippa Roe, Conservative leader of Westminster City Council, told BBC Radio 4's The World At One programme: "I know the vast majority of Romanians and Bulgarians planning to come to the UK are planning to work and contribute to society here.
"I think the fear that everybody faces, which is reflected in the media, is those that come to Britain and either fail to find jobs and therefore fall back on our welfare system, or those who deliberately come here to pickpocket and aggressively beg.
"We have seen in the past 18 months, particularly the Roma in central London, causing a massive amount of disruption and low-level crime which has made a very negative impact on our communities.
"It's this minority one is really concerned about but it is this minority that has this really big impact.
"You've only got to wander around Marble Arch at 7.30 in the morning to see the camps.
"We have people walking out of their front door to find people sleeping on their front doorsteps, people defecating on their front doorstep.
"It's extremely unpleasant and it goes with the very aggressive begging and pickpocketing and other sorts of crime in the area which affects both residents and tourists."