Constance Marten, 35, and her boyfriend Mark Gordon, 48, went missing after their car broke down in Bolton on 5 January this year. It is believed Constance gave birth either in or near the car only a day or two earlier.

Neither the baby nor Constance have received any medical attention.

Police say it is unknown if the baby was full-term or has any health issues, which is why officers "remain committed to finding the family".

The last confirmed sighting of the couple was on Sunday 8 January when they walked along Cantercrow Hill, in Newhaven, East Sussex.

Detective Superintendent Lewis Basford said: "We've been working around the clock behind the scenes and we've viewed more than 660 hours of CCTV as part of our efforts to find them.

"They could be absolutely anywhere in the UK, so we need everyone to remain vigilant.

"Information from the public has been vital to our investigation and we have received more than 325 calls to our dedicated incident room - thank you to everyone who has made contact with us so far, we are so grateful to you."

Despite the volume of calls from the public, police have been unable to locate the couple. A reward of £10,000 for any information is still being offered after first being announced in January.

'Genuine concern for health and wellbeing'

DSI Basford explained that the couple were last seen with a blue two-man tent as they were believed to be camping in Newhaven and elsewhere in Sussex.

"I would like to stress that we are not doing this and putting so many resources and efforts into finding the family just to be awkward or to interfere," he added.

"We have a genuine concern for the health and wellbeing of the baby, and Constance and Mark, and it our duty to ensure that they are okay."

Director of midwifery for Barts Health NHS Trust, Shereen Nimmo, also issued an appeal directly to Ms Marten, urging her to get medical checks done on her child.

She stressed that it was "not too late" for the child to have these checks done, but the longer the couple do not access medical care, the more they are "putting [their] baby at risk".

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Where have the couple been spotted so far?

Officers believe that following the car fire, the family travelled to Liverpool and then onwards to Harwich via a taxi at about 3.30am on Friday 6 January.

They have also been seen in east London. On Saturday 7 January, they were spotted carrying bags containing camping equipment - including sleeping bags and two pillows. They spent the next few hours along the Whitechapel Road area, before walking to Brick Lane.

At around 11.46pm, they dumped a number of items, including a pushchair and got into a taxi to Haringey, north London, before travelling to East Sussex in another taxi, in the early hours of Sunday 8 January.

They were seen sheltering from the rain, under an overpass, at about 6.00am on Sunday morning in Newhaven. Around 15 minutes later they walked into the fields beyond Cantercrow Hill.

They are believed to have moved some considerable distance since.

Ms Marten, who is from a wealthy aristocratic family, first met Gordon in 2016. She was a promising drama student at the time.

Since then the couple have led an isolated life, and in September, when Ms Marten was well into her pregnancy, the couple began moving around rental flats.

Gordon served 20 years in a US prison for rape and battery committed when he was 14.