German prosecutors have reportedly written to Madeleine McCann's parents to tell them their daughter is dead, but refused to tell them how they know.

The heartbreaking news was delivered to Gerry and Kate McCann by the team investigating lead suspect Christian Brueckner's role in her disappearance.

Hans Christian Walters, who is leading the German investigation, said revealing the information could derail the probe and refused to even share it with his British or Portuguese counterparts.

"I sympathise with the parents but if we reveal more details to them it might jeopardise the investigation," he said.

"We have concrete evidence that our suspect has killed Madeleine and this means she is dead.

Kate and Gerry McCann received the heartbreaking letter from German prosecutors. Photo / File

"The parents have been told the German police have evidence that she is dead but we have not told them the details."

He said that Maddie's parents had not responded to the letter.

"We of course really consider the fact that it is going to be very hard for the family when we tell them that we assume Madeleine is dead,' he said.

"But we can't say why she is dead – it is more important that we are successful and we are able to get the culprit as opposed to just putting our cards on the table and telling them why we think she might be.

"This is a murder case, not a missing persons case. We have been quite clear throughout we are investigating a murder and we have the evidence for that.

"We can understand the pain of the parents – and they want relief – but it is better for them that we have a clear and successful conclusion to the case.

"To reveal too many details too early would hamper ongoing investigations.

"In Germany we are very reserved, I am not sure how it is in the UK, we keep everything to ourselves until we press charges."

Christian Brueckner is the main suspect. Photo / Supplied

German detectives revealed earlier this week that they believe Brueckner, the prime suspect in the case, may have written letters about her as Portuguese police examine whether her body was dumped in a well.

Detectives probing into the life of Christian Brueckner are hunting for the supposed letters and have asked ex-girlfriends for notes the convicted sex offender may have sent them in the past.

The move comes after it was revealed Portuguese police had been looking at the possibility McCann's body was dumped in wells near Brueckner's old home, according to claims.

Weekly Portuguese newspaper SOL said police had already undertaken some work aimed at establishing whether the missing youngster's body might be in one of them on the basis that she is dead as the German authorities insist.

"In the spirit of cooperation with their German counterparts, the PJ have been undertaking tasks which include work on wells near a house in which Christian Brueckner lived in the south of Portugal.

"The aim is to ascertain whether there exists any possibility the British girl's body could have been left there."