His intervention came hours after the girl's mother said she would support Jeff and Jennifer Bramley - who disappeared with the half sisters the day they were due to hand them back to social services - in their attempts to adopt.
Paul Duckett, the father of five-year-old Jade Bennett, said he had contacted a solicitor and was prepared to go to court. "I am not happy with the present circumstances," he said.
Mr Duckett, 24, added he was certain that Jackie Bennett, Jade and Hannah's mother, would regret her decision to help the Bramleys to adopt her children.
The Bramleys were turned down after social services decided they were too strict. But in an open letter published yesterday, they wrote: "Social services seemed pleased with us. Everything was fine until one day they said we were too safety conscious, saying `no' and `don't' too often to the girls ... we are two good, honest, caring people who are willing to give up our home, our family and friends and jobs to maintain Jade and Hannah's happiness in keeping them with the parents they love."
Ms Bennett had previously insisted she wanted her daughters back but after seeing the Bramleys' letter she said she changed her mind.
Mr Duckett, who said he has been in a stable relationship for the last four years, claimed his former partner was fragile and might have been overwhelmed by the emotional nature of the Bramleys' letter.
"She started off saying she wants them back, but she loves those children and I worry she has been manipulated into saying that she will give them up," he said. Mr Duckett, who runs a computer company, admitted he had not seen his daughter for nearly three years but said he still loved her.
"I tried to keep in touch but it was very difficult because Jackie kept moving around.
"Living with me would be the best solution for her because I am her father and I am not going to walk away." Mr Duckett said he had no sympathy with the Bramleys and did not believe they should be allowed to adopt the children. "They didn't seem to be the caring parents I thought they were."
Cambridgeshire Social Services has refused to elaborate on why the Bramleys were refused permission to adopt the children but a spokesman said: "It is incredibly unusual for a local authority to terminate a placement and the last time we did it was 10 years ago.
"That is an indication of how seriously the situation is taken. There were significant areas of their parenting skills that caused concern and after careful consideration a decision was taken that this would not be an appropriate placement for the two girls."
It is 17 weeks since the Bramleys disappeared. They took only pounds 5,000 with them and have made no further withdrawals since. Their car, found in York more than two weeks ago, has been impounded by the police. Last week there was a sighting reported on a train in North Yorkshire. Their letter would seem to be the first step towards admitting that they want to come home. But even when they do turn up, the future of the children will still hang in the balance as the adults fight over where they should live.
Police were yesterday checking reported sightings of the Bramleys in Nottingham, where the letter was posted, and CCTV footage from a surf shop in Skegness where a store owner says the Bramleys were browsing on January 2.Reuse content