Campaigners accused Britain of betraying vulnerable children Wednesday after a scheme to take in unaccompanied young migrants from Europe was limited to 350 people -- 200 of whom have already arrived.

The number falls well short of the 3,000 proposed by the original advocate of the scheme, opposition Labor politician Alf Dubs, 84, who himself arrived in Britain as a child fleeing the Nazis.

The government did not specify how many children it would take in when it announced the plan last year, amid widespread concern about the fate of Syrian refugees fleeing to Europe.

Immigration minister Robert Goodwill told MPs on Wednesday that the total number would be 350, including "over 200 children already transferred under section 67 (of the Immigration Act) from France".

This refers to children brought over the Channel from the "jungle" migrant camp in Calais on the northeast French coast.

"We will announce in due course the basis on which further children will be transferred from Europe to the UK... to the specified number," he said.

The decision was based on an assessment by local authorities across Britain on how many children they could manage, with a government spokesman stressing their often "difficult needs".

But the announcement was widely condemned, with Dubs himself warning that to shut the scheme would be "shameful".

"At a time when Donald Trump is banning refugees from America, it would be shameful if the UK followed suit by closing down this route to sanctuary for unaccompanied children just months after it was opened," the peer said.

"During the Kindertransport, Sir Nicky Winton rescued 669 children from Nazi persecution virtually single-handedly. I was one of those lucky ones.

"It would be a terrible betrayal of his legacy if as a country we were unable to do more than this to help a new generation of child refugees."