TWO of Britain's most dangerous paedophiles were last night at the centre of the Madeleine McCann investigation.

Depraved Charles O'Neill, 48, and William Lauchlan, 34, were jailed last June for sex attacks on children and the murder of a mum-of-three who threatened to expose them.

O'Neill was sentenced to a minimum of 30 years and Lauchlan to 26. 

But now it has emerged the monsters, who were also gay lovers, were touring Spain, and possibly Portugal, on false passports when Madeleine vanished on May 3, 2007.

Police have already spoken to Lauchlan about Madeleine and an alleged child smuggling operation on the continent.

Detectives are said to be keen to now talk to O'Neill because of his resemblance to a thin, spotty suspect seen

hanging around the holiday complex in Praia da Luz on the Algarve shortly before the three-year-old went missing.

It is the first time British police have interviewed anyone in connection with the case but Leicestershire Police, the McCanns' local force, said they could not comment because the inquiry was being led by officers in Portugal.

A senior officer said last night: 'The whole Madeleine McCann inquiry is being kept extremely tight at the very highest level.'

The development came as, for the first time, Madeleine's mum Kate revealed she thought about killing herself in the days after her daughter's disappearance.

In her book, due to be published on Thursday, Madeleine's eighth birthday, the 43-year-old writes: 'I had an overwhelming urge to swim out across the ocean, as hard and as fast as I could; to swim and swim and swim until I was so far out and so exhausted I could just allow the water to pull me under and relieve me of this torment.

'I wasn't keeping that desire to myself, either. I was shouting it out to anyone who happened to be in the room.

'I was crying out that I could see Madeleine lying, cold and mottled, on a big grey stone slab.

'Seeing me like this must have been terrible for my friends and relatives, particularly my parents, but I couldn't help myself.'

O'Neill and Lauchlan, originally from Largs, Ayrshire, abused children in Britain and abroad.

They also strangled Allison McGarrigle, 39, who disappeared in 1997 after she threatened to tell police about them abusing a boy.

They have also been linked to the disappearance of Yeremi Vargas, who was seven when he disappeared while playing outside his Gran Canaria home in 2007. 

The child is still missing and his mother remains convinced the Scottish killers are responsible.