The UK Border Agency is to set up a £4m "reintegration centre" in Afghanistan so that it can start deporting unaccompanied child asylum seekers to Kabul from Britain, the Guardian can disclose.

The terms of the official tender for the centre show that immigration officials initially hope to forcibly return 12 boys a month aged under 18 to Afghanistan and provide "reintegration assistance" for 120 adults a month.

Home Office figures show there are more than 4,200 unaccompanied child asylum seekers in Britain, with most being supported in local authority social services homes. Those from Afghanistan are the largest group. Of the 400 minors claiming asylum in the first three months of this year, almost half were Afghans.

A decision to start deporting Afghan child asylum seekers who arrive in Britain alone would amount to a major shift in policy. Up until now, child protection issues and an undertaking that failed child asylum seekers would be returned only if adequate reception and care arrangements were in place for them on arrival have blocked returns.

The British plans form part of a wider European move to plan the return of unaccompanied migrant children to Afghanistan. Norway has also announced plans to open a reception centre in Kabul. Sweden, Denmark and the Netherlands are also reported to be preparing to return Afghan children to Kabul.

Home Office ministers backed a new EU action plan on unaccompanied minors last Thursday when justice and home affairs ministers met in Luxembourg. The plan included support for countries of origin "creating reception centres that can provide care for minors when the family cannot be found".

The development has sounded the alarm among refugee welfare and human rights organisations that the EU has given a green light to move ahead with deportations with too little being done to guarantee the safety of the children sent back.

Simone Troller of Human Rights Watch said: "Before deporting vulnerable kids to places like Afghanistan, EU governments need to make sure it is in the children's best interests."

She said that the British government had circulated a policy paper on unaccompanied minors in February during a Brussels workshop that called for an "EU-wide presumption" that a child's best interest was to return. It also argued against formal safeguards such as guardianship as "immensely expensive to put in place".

The details of the UK Border Agency tender document issued in March say the £4m contract is intended to "provide reintegration assistance for Afghans whose return home is being enforced by the UK government because they have no right to remain in the UK". The terms of the contract say that 12 young Afghan males aged 16 and 17 are to be sent back a month. They will be accommodated until they are 18, with adults providing supervisory care. The numbers imply that up to 150 teenagers would be sent back in the first year.

The centre will also provide reintegration assistance, including vocational training, business start-up grants and short-term accommodation for 120 deported adults each month.

A decision to start sending back 16- and 17-year-old asylum seekers to Afghanistan would also render academic many "age disputed" cases in which the UKBA claims adults are posing as children to avoid removal.

The immigration minister, Damian Green, said tonight: "No one should be encouraging children to make dangerous journeys across the world. Therefore we are looking to work with other European countries, such as Norway, and valued international partners, such as Unicef, as well as the Afghan government, to find ways to help these young men and women in their home countries and to return those who are in the UK safely to their home nations with appropriate support once they arrive."

The Refugee Council said ministers should urgently review the plans to start removing unaccompanied minors to countries that are not safe. Its chief executive, Donna Covey, said: "There has been little said about how these children would be kept safe … if they have no family to whom they can be returned safely, should they be returned at all? There are serious questions to be raised about the quality of decision-making on the cases of unaccompanied children. The money would be better spent improving the way that children's claims are assessed, so that we can be sure we never put them in danger."