Dozens of children – sent from Afghanistan by their parents in the hope of finding a better life in Britain – are sleeping rough on the streets of Calais following the demolition of the town's last remaining asylum camp, an investigation by The Independent has found.

In a two-day visit to the Pas-de-Calais region, at least 30 children between the ages of 10 and 16 were identified sleeping under canal bridges and in the forests surrounding the town. The boys are reliant on the people-traffickers who plan to stow them away on to trucks into Britain. Most of the children were sent abroad by parents who paid tens of thousands of dollars to trafficking cartels who falsely assured them that their children would be safe in Europe.

Aid agencies say the French authorities are aware of their presence but no attempt has been made to take them into care.

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Under Article 20 of the United Nations Convention on the Rights of the Child, which France ratified in 1990, countries are obliged to provide "special protection and assistance" to any children who have been deprived of their family.

Instead, charity workers and migrants say riot police have resorted to increasingly brutal tactics to ensure that the refugees do not build any new permanent settlements in a bid to discourage new arrivals. Officers now regularly move them on at night, often confiscating the blankets that are given to them by charity workers. Pierre de Bousquet, the man in charge of police operations in Calais, and Natacha Bouchart, the town's mayor, refused to comment on the allegations.

Riot police bulldozed the encampment known as the Jungle last month, arresting more than 250 migrants, half of whom were minors. They have also dismantled a number of derelict houses in the centre of town that were being used by African migrants.

France shrugged off accusations that the camp's closure would prove to be a humanitarian disaster by arguing that the Jungle had become a magnet for traffickers. But critics say the camp's closure has simply forced a vulnerable group of people back on to the streets.

Hundreds of refugees are now as a consequence living in conditions that are even worse than when the Jungle existed. Most sleep under a series of bridges on the eastern edge of Calais, or in the forests close to where the Jungle once stood. Two taps that the migrants used for fresh water near the Jungle have also been removed, forcing many to drink from sluice channels despite signs stating that the water is not clean.

Charity workers from La Belle Etoile and Association Salaam, which feed the migrants, say the number of refugees turning up for the two daily soup kitchens has decreased to approximately 250 per day, down from a high of 850 earlier in the summer. But many have simply moved to a number of smaller camps further up the coast that are run by trafficking gangs and provide them with even less security.

Nadine Bouteille, from La Belle Etoile, said: "We are particularly worried about the arrival of winter, especially for the young and wounded. Clearing the Jungle has only made the situation worse. Traffickers never bothered staying in the Jungle. If [the authorities] were serious about cracking down on traffickers they would start looking in ordinary houses or in Calais hotels, not spending their nights chasing innocent refugees."

One camp located on the outskirts of Dunkerque, 45km to the east of Calais and hidden in a small forest, is occupied by 35 Afghan refugees. Two of the camp's residents were just 14 years old. Unlike in Calais, these migrants said they were largely left in peace by the authorities. Another camp down the road, near a town called Loon-Plage, contained 30 migrants. Both were being run by Kurdish people-traffickers who, quick to cash in on the refugees' increased determination to cross the Channel, have put their prices up from about €1,000 (£895) to €2,500.

Dan Hodges, from Refugee Action, called on the French government to help the hundreds of migrants living rough in northern France. "Whilst it was possible to sweep away the camp, it has not been possible to sweep away the problem," he said. "Rather than bringing these asylum-seekers into the French immigration system, the authorities are instead pushing people further into the arms of smugglers. It is an international disgrace that France is treating asylum-seekers, including hundreds of children, like refugees from the Dark Ages."

William Spindler, a spokesman for UNHCR – the UN refugee agency – in Paris said: "There has been a noticeable shift in police tactics over the past few weeks. The refugees are now living in even worse conditions than they were and this is very concerning, especially for the children."