Children could be at more risk from paedophiles under plans to merge a child protection body with the new national crime agency, ministers were warned yesterday.

The parents of Madeleine McCann joined police chiefs and Sara Payne in voicing concerns over the plans for the Child Exploitation and Online Protection Centre (Ceop).

Jim Gamble, the centre's head, this week announced he was quitting in protest at the merger, which he believes would leave children more vulnerable.

The growing condemnation last night piled pressure on Theresa May, the Home Secretary, but she stood firm in her intention to merge Ceop with the National Crime Agency.

Kate and Gerry McCann urged ministers to remember the importance of the "invaluable work" carried out by Ceop while Alan Johnson, the former Home Secretary, said the move would "harm child safety networks".

Sara Payne, campaigner and mother of murdered eight-year-old schoolgirl Sarah, said she was "disgusted" by the Government's actions and the Association of Chief Police Officers (Acpo) said the body should remain stand alone.

Mr Gamble quit as Ceop chief executive on Monday warning the plans were not in the "best interest" of vulnerable children.

His decision sparked widespread criticism of the Home Office and concerns that children will not be as well protected from those looking to groom them online.

Mr and Mrs McCann said in a statement: "Knowing how committed Mr Gamble is to this cause, it is extremely saddening that he feels unable to continue to lead Ceop, apparently as a consequence of the proposed Governmental changes.

"In this challenging economic climate, we urge the Government to remember the value of our children and the importance of the invaluable work which is necessary to protect them against the devastating crimes of child abduction and exploitation."

Mrs Payne, Shy Keenan and Fiona Crook – who jointly set up the campaign group the Phoenix Foundation – said: "We cannot begin to describe how disgusted we are with our own Government for betraying him and for betraying all of our children.

"This cannot be allowed to happen, we must stand up and fight, we must do what is right for the protection of our children against the crimes of paedophiles."

Ms Keenan added: "The Government has already severely diluted child protection services. Deconstructing one of this country's most effective, cutting-edge, child protection-focused centres is not in the best interest of our children, full stop."

The Association of Chief Police Officers (Acpo) said it was "in firm support" of Ceop remaining as a stand-alone agency.

Maggie Atkinson, the Children's Commissioner for England, warned that Ceop could be lost in the new organisation and that vulnerable young people would be less willing to approach it for help.

Ceop was set up in 2006 tasked with tracking online paedophiles and bringing them to court.

It is currently affiliated to the Serious Organised Crime Agency (Soca).

But under Government plans, the agency will become part of a new National Crime Agency in 2013.

As Ceop chief executive, Mr Gamble led calls for Facebook to set up a "panic button" to give reassurance for young users. The application has been downloaded tens of thousands of times since being launched in July.

A Home Office spokesman denied there were any plans to undermine the work of Ceop, saying child protection "will always be an absolute priority for the Government".

"We propose to build on Ceop's invaluable work tackling sexual exploitation of children by placing it centre-stage in the fight against all forms of serious and organised crime," he said.

"We are determined that plans for the National Crime Agency will enhance all our capabilities, and discussions are ongoing about how that will be done."

The spokesman added: "Ceop already delivers excellent work as part of the Serious Organised Crime Agency and the Government is clear this work must continue."