The mother of a missing aristocrat who vanished more than a month ago with her newborn baby and convicted sex offender boyfriend has issued an emotional open letter to her daughter.

The Metropolitan Police have been searching for Constance Marten, 35, and her partner Mark Gorden, 48, and their baby and believe they are camping in the Sussex countryside.

Virginie de Selliers, Miss Marten’s mum, has pledged to stand by her daughter and grandchild, telling her: “You are not alone in this situation. We will support you in whatever way we can.”

Police believe the couple are sleeping rough in a blue tent and fear for the safety of the baby, who has not had any medical attention since birth in early January.

In the letter, sent to the PA news agency by a representative for the family, Mrs de Selliers writes: “Open Letter to My Darling Daughter Constance.

“I know you well enough; you are focused, intelligent, passionate and complex with so much to offer the world. So many of your friends have come forward to say such positive things about you, assuring us of their warmest love and support for you and your family.

“You have made choices in your personal adult life which have proven to be challenging, however I respect them, I know that you want to keep your precious new-born child at all costs.

“With all that you have gone through this baby cannot be removed from you but instead needs looking after in a kind and warm environment.

“I want to help you and my grandchild. You deserve the opportunity to build a new life, establish a stable family and enjoy the same freedoms that most of us have.

“Constance, I will do what I can to stand alongside you and my grandchild. You are not alone in this situation. We will support you in whatever way we can.

“I am ready to do what it takes for you to recover from this awful experience so you can thrive and enjoy motherhood.

“I love you and miss you, Mum xx.”

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It is the first time that Miss Marten’s mother has said anything publicly about her daughter’s disappearance.

The missing couple have so far avoided being traced by the police by moving around frequently and keeping their faces covered in CCTV images.

The new photograph captured by CCTV and released by the Met Police shows the pair in Newhaven.

The trio were dropped off in a taxi at 4.56am on Sunday, 8 January just outside the entrance to the Newhaven Ferry.

They then walked to where the A259 crosses over the B2109.

At around 6.15am on Sunday, January 8, they walked along Cantercrow Hill into the fields beyond as the latest CCTV pictures show.

The road is a dead end but leads onto paths up into the South Downs.

A £10,000 reward is on offer for information that leads to them being found safe.

Investigators said that in the months before they vanished they built up a significant amount of cash and used it to pay for places to stay and taxis around the country.

The couple travelled from Bolton to Liverpool, then to Harwich in Essex, then to east London and then to Newhaven, where they were seen near the ferry port on January 8.

In an update on Sunday, the Metropolitan Police said they believe the family may be camping in Sussex.

Anyone with information can call the incident room on 020 7175 0785 or Crimestoppers anonymously on 0800 555 111.