The union wants security at courts to be stepped up

Napo leader Harry Fletcher, who did not accuse anyone specifically, said court staff had been threatened and names and addresses published on websites.

Banners denouncing staff as child abusers had been unfurled on court buildings, Mr Fletcher said.

The union wants the government to make sure the police take effective action.

It also wants security at courts to be stepped up.

The union has sent a dossier to Children's Minister Margaret Hodge, which it says outlines dozens of incidents of verbal abuse and physical threats.

Fathers rights' campaigners say the family courts system is inherently biased against men, it unfairly denies contact to men and where it does make contact orders, it does not properly enforce them.

But Mr Fletcher said some fathers were unfairly targeting his members.

He said: "They are being increasingly targeted by militant fathers' groups who are upset because they think there is some kind of systematic bias in the system against men.

'Extremely worrying'

"The intimidation is extremely worrying. It has involved verbal abuse, physical threats, names of staff have been published on websites, staff have been told their addresses are known and they will be followed.

"Offices have been daubed with paint, locks have been superglued, windows have been broken, we even had banners unfurled on our buildings denouncing family court staff as child abusers."

Mr Fletcher said that of 30,000 cases dealt with every year, only 0.8% saw contact with the father refused.

It (the Napo report) plays into the hands of the extremist minority who are on the fringes of what we do Fathers 4 Rights founder Matt O'Connor

Fathers 4 Justice, which has run a high-profile campaign against the court system, said intimidatory tactics went against its policy and called the Napo report "irresponsible".

"It is irresponsible because it plays into the hands of the extremist minority who are on the fringes of what we do", said the group's founder Matt O'Connor.

"It's like a red rag to a bull to these fringe groups".

'Condemn'

He said Fathers for Justice had already been in negotiations with the Children and Family Court Advisory Support Service (Cafcass) about a way forward.

The fathers' rights group had asked Napo if it could see its evidence and work with them, he added.

He appealed again for this to happen.

He said intimidatory tactics went against the group's terms and conditions of membership, which also ruled out sexist and racist behaviour.

"We have never named and shamed or targeted individuals, and that is something we condemn," he said.