The son of missing mum Sarah Wellgreen has penned a heartbreaking birthday message message to his beloved mother as he turns 23 today.

Sarah has been missing since October 9 and was last seen in the Bazes Shaw area of New Ash Green. Police are now treating her disappearance as a potential murder.

Her ex-partner Ben Lacomba is set to stand trial in June after being charged with her murder, a charge to which he has pleaded not guilty.

MORE: Sarah Wellgreen case: Ben Lacomba pleads not guilty to murder

In his emotional note posted on Facebook, Lewis Burdett calls on "this nightmare to end" as he opens up about his heart-wrenching experiences over the past four months.

He wrote: "Dear Mum. I wanted to write to you today, it’s my birthday...but you know that.

"I’m trying to carry on and be strong but days like this really hit me. I just wish you were here.

"I keep looking at my phone waiting for you to ring or message even though I know you can't and you can't be with me.

"I’m finding it hard not being able to talk to you, to tell you our news, what we're all doing. I even pick my phone up without thinking to ask you something.

News Shopper:

"I know I was lucky having you for so many years but I just didn’t think it would end so soon.

"So when it’s quiet I just sit and have a cup of tea and a chat, I know you can hear me.

"I will love you forever Mum.

"I’m going to share this letter with all the people helping to find you. So many people care about you and your in their thoughts and that helps me know we’re not alone."

MORE: Sarah Wellgreen: Volunteers blast claims that they were 'stood down' by police investigators

Since Sarah's disappearance, police have searched woodland near Bluewater shopping centre and areas in Greenhithe near the River Thames.

Divers have entered the River Darent in Dartford, while police and council workers have searched drains and the fire service has provided a drone.

Local volunteers who had been leading the public searches for the missing mum-of-five recently launched a fundraiser for her family, with the aim of raising £50,000.

The search for Sarah Wellgreen is ongoing.