Members of the Home Affairs Select Committee found detention periods of up to 61 days were not uncommon for some youngsters, with the average term being more than a fortnight.

Chairman Keith Vaz said: “It is not acceptable that we are detaining so many children for such long periods of time – these children have done nothing wrong, they should not be being punished.

“It must always be absolutely the last resort to keep a child detained for any length of time.”

Nearly 1000 children a year are detained in UK Border Agency (UKBA) detention centres, the committee said, at an average cost of £130 a day.

They are mainly held with their asylum-seeker families before being deported.

The committee said the rationale for detention was the risk of absconding, but found there was no evidence of families “systematically” disappearing.

Its report said: “The UKBA should make every effort to reduce the need to detain small children for sustained periods of time ... we insist that this power be used only sparingly, as a last resort and for the shortest possible time.”

The risk of families absconding was low, it added, and both in “moral and financial terms, was a price worth paying”.

SNP MP Pete Wishart recently discovered that more than 100 children had been held at the Dungavel Immigration Removal Centre in Lanarkshire over the past year and is now investigating how long the children were held for and what happened to them after they left Dungavel.

His SNP colleague Anne McLaughlin is currently championing the cause of Florence Mhango and her 10-year-old daughter, Precious, who was detained at Dungavel and now at Yarl’s Wood.

Ms McLaughlin said: “The raw numbers in this report highlight the unacceptable number of children being detained with one sample showing up to one-third had been detained for more than 30 days when there is no real risk they will abscond.

“The experience of Precious Mhango shows how distressing it can be for children to be detained. Precious goes to school in Glasgow, she belongs in Scotland but the indiscriminate asylum system has seen her detained twice when there is no danger that her family would abscond.

“Her unnecessary detention is exactly what this report is talking about.”