Fresh details have emerged about the removal of three eastern European children from their foster parents that cast doubt on claims the decision was politically motivated and taken purely because the couple were members of Ukip.

The removal of the children from the Ukip-supporting foster couple in Rotherham caused outrage across the political divide, prompting questions in parliament and providing Ukip with a boost before Thursday's byelections. The Rotherham foster parents say the only reason they have been given for the abrupt removal of the children two weeks ago was their membership of Ukip.

But sources close to the case have told the Guardian there were multiple legal and social reasons why the council wanted to ensure the children be placed with foster parents who spoke their own eastern European language.

The placement with the Ukip-supporting foster couple was not intended to be long-term. It was an emergency move amid allegations that the children's birth father had sexually abused two of his daughters and had held a knife to his wife's head while she was holding their baby. According to the birth parents, the children were taken in a raid by police and social workers earlier this year.

There were also fears the children's birth parents knew or might be able to find out where the foster parents lived. Though both the birth mother and father claim to continue to have supervised contact with some of their other children, it is believed social workers do not want the parents to know exactly where the children are living because of safety concerns.

There was also tension between the council and Rotherham's 3,500-strong Roma population, almost all of whom moved to the South Yorkshire town since EU enlargement in 2004.

A meeting was held after Czech and Slovak media reports claimed that Roma children were being "sold" to white couples in Britain under the guise of adoption. A community worker present at that meeting said relatives of the children in the Ukip foster case had harangued the council , asking why the children had been taken away and stripped of their language and heritage.

The children's parents, Roma who moved to Rotherham, told the Guardian they have been in and out of family courts trying to get their children back after they returned from an earlier foster placement unable to speak their mother tongue.

The parents claim the children had been taken away after one of their sons had been found wandering the streets of Rotherham at 2am shortly after arriving in Britain. They said social workers also had concerns about overcrowding: at the time the family were living in a house that was infested with mice.

Social workers wanted the children to be permanently removed from their parents, concerned after discovering the father had been accused of sexually abusing his wife's daughter from a previous relationship. A social worker from Rotherham Borough Council had travelled to their hometown to try to find out more about the family, and co-operated with the local social services department.

But a family court judge ruled three of the children should be returned to the parents after the birth parents successfully argued that the council had failed in their duty to ensure the children enjoyed the linguistic right to learn and speak the language of their birth. In an interview, the father, along with his wife, told the Guardian the claims of sexual and physical abuse were unfounded. "We just want the children back and the social services to leave us alone. We just want to live as a normal family," said the father.

He said he was "disgusted" to learn that his children had been placed with foster carers who were members of a party opposed to eastern European immigration.

The man said he only found out his family was at the centre of a political and media storm when his barrister told him at a court hearing this Wednesday.

Rotherham council refused to answer questions about the case. "These children are the subject of court proceedings and in these circumstances the council is not able to make any comment," said Jacqueline Collins, director of legal and democratic services.

Joyce Thacker, the council's director of children and young people's services, was criticised last weekend when she appeared to admit that Ukip members might not be suitable foster parents for non-British children. "These children are not UK children and we were not aware of the foster parents having strong political views. There are some strong views in the Ukip party and we have to think of the future of the children," she said last weekend.

Ukip leader Nigel Farage has demanded an apology for the foster parents from Rotherham Borough Council. "It is not acceptable for the council to prevaricate on this, we want to see action now," he said earlier this week. "Ordinary people all over the country – and most senior politicians, with the notable exception of David Cameron – are absolutely incensed about this case. Rotherham council should apologise immediately, and heads should roll.

"Joyce Thacker should surely go, along with any other senior managers who have endorsed this bigoted and discriminatory policy."