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The National Missing Persons Helpline told BBC News Online all parents tell them it is the "not knowing" which is the worst.

"If someone you love goes missing you can't help but keep running through the worst possible scenarios in your mind," said spokesperson Jessica Prasad.

"You really don't get any peace until there is some kind of closure.

"Some people say that even if they learn their child is dead that at least gives them some kind of closure, so they can move on and go through the mourning process.

"Otherwise they can be stuck in a kind of limbo forever."

Scanning the TV

She said all parents found it extremely difficult to get on with their lives while waiting to find out what had happened to their child.

"Some parents are afraid to leave the phone or go out for too long, because that could be the hour or 10 minutes their child decides to call.

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"Others say every time they watch television they're scanning the audience in case they see their relative.

"Even in the streets they're constantly scanning people's faces to see if they can see them."

Some parents have to go for years not knowing what has happened to their child, she said - and every time a body was found, they would frantically think: "Is that my child?"

Eventually some people find it easier to declare their relative legally dead - which can be done after seven years.

"It's quite a final thing to do but it can provide some sort of closure for people who just can't live with this constant situation of not knowing.

"But other people would find this impossible because they're still clinging onto any shred of hope that one day the person will be found."

Media appeals

Ms Prasad said there were few things parents could do to ease the pain - although appealing for sightings and making sure they could be contacted helped.

"They can in the initial stages contact local or national media if they want to - that depends very much on the family - as an appeal to their loved one, and also as an appeal for sightings.

"Then they can register the missing person with the police and with the National Missing Persons Helpline, so that if any sightings of their relative are made, they can be contacted."

As for support, most parents in the Dowlers' situation would have a police liaison officer to talk to.

Support group call

And people registered with the National Missing Persons Helpline would have a case worker they could call to talk to, to find out if there is any information or just to get things off their chest.

Ms Prasad said two things could be done to make life slightly more bearable for parents in the Dowlers' situation.

One was the funding of a mutual support group where parents of missing children could contact and help each other.

The other was Home Office funding for a comprehensive national police database of all missing persons, to speed up the process of identification and contact.