MPs have warned the Home Office that it cannot ignore the growing numbers of child refugees returning to sleep rough on the edges of Calais and Dunkirk after the closure of the so-called Dubs scheme, which gave lone children a safe route to asylum in the UK.

Volunteers said the UK government’s decision to take no more lone child refugees from Europe had been a fillip to traffickers, who were emboldened by the desperation of teenagers rejected by official schemes.

Visiting the Grand-Synthe camp close to Dunkirk on Monday, the Labour MP Yvette Cooper said she would hold an emergency inquiry as chair of the home affairs select committee on the effects of the closure of the Dubs scheme.

On Thursday the House of Commons is to debate the effective closure of the scheme. MPs could also bring amendments or private members’ bills as part of cross-party efforts for a change in policy.

Cooper said MPs felt misled by the premature closure of the scheme. “It has only been open for six months, it takes councils time to set up systems,” she said. “It feels like the danger is we are going back to square one, with teenagers and children back to being at risk of traffickers.”

The Conservative MP Heidi Allen, who has worked closely with Cooper to advocate for child refugees to be given asylum in the UK, said the evidence she had seen in Dunkirk and Calais meant she was unconvinced by the home secretary Amber Rudd’s claim that the Dubs scheme was acting as a pull factor.

“Desperation is what the traffickers want,” Allen said. “They say, ‘Every route has been closed, but I can help you.’ Volunteers have no hope to offer them, to make them go back to the official centres.”

When the Guardian visited Grand-Synthe with the MPs, volunteers said they have details of almost 100 under-18s, many of whom believe they are eligible for transfer to the UK because of family reunification laws or would have been eligible under the Dubs scheme before its cancellation.

Many hundreds more are sleeping rough in other small unofficial encampments, about 200 of them teenagers, without any form of shelter apart from a donated sleeping bag.

The majority are former residents of the now-closed Calais refugee camp and were moved to official French migrant centres. The teenagers in the Grand-Synthe camp who spoke to the Guardian said they were promised a legal route for asylum in the UK but now, either frustrated by delays or unable to understand why their applications had been rejected, they had now returned to the French coast to try to clamber on to lorries or trains.

One Afghan boy, Usman, who said he was 13, said he had applied to join his uncle in London under the Dublin regulation, the law which reunites lone minors with family members in other EU nations. Usman said he had been told his application had been turned down, without any reason given or any paperwork.

Turning his palms upward, he showed the MPs his palms, which were bruised and grazed from attempting to jump on to lorries. “I know it is dangerous,” he said with a shrugged.

Life was better in the official French migrant centres, he added. “There was a shower every day, here once a week, maybe not at all. But there is no hope.”

No hope of getting to the UK, he meant. With his red drawstring bag across his chest, Usman is packed ready to try the lorries on Monday night. “Here there is a hope, yes it is hard but there is a hope,” he said. “In the UK there is my family, here nothing.”

Volunteers from Help Refugees and French charity L’Auberge de Migrants said the smaller camps springing up around the Calais area were extremely dangerous, without any of the registration or welfare systems that had developed around the Calais refugee camp by volunteers until it was dismantled in October.

In Grand-Synthe, numbers have increased so rapidly that at least 90 people are now sleeping on the floor of the camp’s kitchen and dining area, with refugees forced to sleep in shifts because of the shortage of space, and volunteers saying there is a growing problem of scabies. Piled in the corner are dishes encrusted with mould, cleaned only with a filthy rag. Refugees brandished their camera phones, showing pictures they had taken of cockroaches and maggots.

Save the Children’s Al Russell, working in the Dunkirk camp, said having safe and legal routes to the UK was not aiding the traffickers. “It disrupts traffickers,” he said. “Now the traffickers can tell them they have no way to the UK apart from to pay.”

Margot Bernard, child protection coordinator at L’Auberge des Migrants, said: “The Dubs scheme meant we could persuade children to wait, to go to school, to stay in a warm and safe place. For those who really have a set mind on the UK, even if it’s the best thing for them to stay in France, this was a way for them to go safely.”

On Monday, the Home Office said it was prepared to review the asylum applications of a number of children who had returned to the site of the original Calais camp. That decision will not help those who have no relatives in the UK.

The amendment, proposed by former Kindertransport refugee Alf Dubs, now a peer, and attached to last year’s Immigration Act, was expected to bring about 3,000 children to the UK. However, the immigration minister Robert Goodwill told MPs in a written statement this month that just one further group of 150 child refugees would be brought to Britain, a total of 350.

Cooper said the growing numbers of children returning to the area was disheartening after the closure of the Calais camp, when MPs had been led to believe a proper system of assessment had been put in place.

“It felt like there was a brief period for a few months when Britain and France were working closely together, it was hand-to-mouth as they were clearing the Calais camp, but they got a lot of the teenagers to safe accommodation,” she said. “But now it has started to fall apart.”

Allen was more blunt. “Has it really fallen apart? Or did we only want it to work a bit? To me, it feels like it was all a gesture,” she said.

A government spokesperson said: “We are committed to supporting vulnerable children who are caught up in conflict and danger. Thanks to the goodwill of the British public and local authorities in the last year alone, we have provided refuge or other forms of leave to more than 8,000 children.

“Our commitment to resettle 350 unaccompanied children from Europe is just one way we are helping. We have also committed to resettle up to 3,000 vulnerable children and family members from the Middle East and north Africa region and 20,000 Syrians by the end of this parliament. We have a proud history of offering protection to those who need it and children will continue to arrive in the UK from around the world through our other resettlement schemes and asylum system.”

