Constance Marten and Mark Gordon have been further arrested on suspicion of gross negligence manslaughter, police have said.
They were arrested in Brighton on Monday on suspicion of child neglect after being spotted by a member of the public just before 9.30pm.
Police said more than 200 officers were involved in an urgent operation to find the baby and have been searching allotments and woodland in the city.
Helicopters, sniffer dogs and drones are also involved.
It’s believed the couple have been living rough in a tent.
In an update late this afternoon, officers said the couple had also been arrested on suspicion of gross negligence manslaughter.
Please use Chrome browser for a more accessible video player
Police search for baby in the area around Brighton
Marten, 35, and Gordon, 48, were reported missing on Thursday 5 January, after their car caught fire on the M61, near Bolton.
They were spotted numerous times around the UK in the days that followed, including in Liverpool, Essex, south London and East Sussex.
Concerns about the baby have been growing as it’s not thought to have had any medical care.