Unofficial migrant camps are likely to grow in northern France in the wake of next week's planned demolition of the "Jungle" shanty-town in Calais, British aid volunteers fear.
Bulldozers will move in on the slum on Monday, with the French authorities saying the estimated 6,500 migrants camped there will be relocated to reception centres across France.
But Care4Calais founder Clare Moseley said that following the part-dismantlement of "the Jungle" in February, serious failings were reported at some of the centres migrants were sent to.
She said there was a lack of basic essentials, including access to interpreters and legal advice. And she added: "We are also concerned that unofficial camps in the north of France will now grow.
"These suffer from a severe lack of infrastructure, no running water, toilets or medical facilities, and so where possible we will also direct aid to these areas."
As France gears up for next year's presidential election, French president Francois Hollande has appeared keen to adopt a firmer stance and finally close the camp.
It has become a symbol of his government's failure to tackle Europe's migrant crisis and a target of criticism from conservative and far-right rivals seeking to unseat him.
Amid reports that migrants may attempt last-ditch bids to cross the English Channel, Kent Police has said it is braced for any fall-out from the demolition.
The force has said it is "monitoring events" in northern France as notices started to go up in the camp alerting migrants to the imminent clearance.
Unicef UK said hundreds of children in Calais were still waiting to reach safety.
Its deputy executive director Lily Caprani said: "Once the demolition starts there are no second chances. If it results in a single child going missing, or forces them into the hands of smugglers and traffickers, then we will have failed them.
"The authorities must prove they have learnt the lessons from last time and keep every child safe throughout this process.
"During the last demolition, which saw people scattered by tear gas and rubber bullets, more than 100 children went missing because it began before their safety was guaranteed.
"We've seen the Home Secretary achieve real progress by bringing some of the vulnerable children to the UK, but there are hundreds more in Calais still waiting to reach safety. The UK has made a commitment to bring these children here, and the demolition doesn't change that."
Dover Conservative MP Charlie Elphicke said the camp must never be allowed to re-emerge again.
He said: "The Jungle must be fully dismantled - never to return. This time they need to see it through. We must end the Calais migrant magnet.
"Border security should be stepped up while the Jungle is being cleared. Tourists and truckers at Calais must be protected from attack by people traffickers armed with petrol bombs and machetes.
"We need more investment in intelligence and border security at Dover - to wage war on the people traffickers. Britain must lead the way in ending their evil trade of modern slavery."