A large-scale search of rough terrain for missing Jay Slater is set to begin after Spanish police put out a call for volunteers to help.

On Friday the Guardia Civil formally invited voluntary organisations and members of the public with expertise in difficult conditions to come to northern Tenerife.

Mr Slater, 19, has not been heard from since the morning of 17 June, after he told a a friend he was lost in the mountains and had 1% battery on his phone.

The Spanish authorities have deployed helicopter crews, specially trained search dogs and drones but after 12 days of searching no trace of the teenager has been found.

The search will focus on the ravines and trails around the village of Masca in the Rural de Teno national park, and will start from the Mirador de la Cruz de Hilda restaurant at 9:00 BST.

Mr Slater had been attending the NRG music festival and his friends said they were out in the tourist hotspot of Playa de las Americas when he was seen getting into a car with two British men he met earlier in the night.

He was later tagged in a photo posted on Snapchat at 07:30 BST at an Airbnb in Masca, which was reportedly being rented by the two men.

Best friend Brad Hargreaves told ITV he received a video call from Mr Slater a short time later.

He said during that call Mr Slater appeared to slide off a designated path, with the video showing his feet on rough ground.

However, Mr Slater then said he had to go as he was receiving another call.

Mr Hargreaves said: “I don’t know how or what has gone on there but he’s gone off and rang me halfway to their house saying I’m staying here and I’ll be back the next day.

“He’s rang me walking down the mountain and he just says he’s walking home.

“At the time I didn’t think anything of it I just thought he was going to get a bus home or a taxi home because that’s what he says he is going to do.

“Next thing you know his phone dies and it’s 10 days on now and nothing since.”

Shortly after 08:30 BST he called another friend on the holiday with him, Lucy Law, and said he did not know where he was, needed water and had 1% battery before the call cut off.

Mr Slater’s mother Debbie Duncan previously told the BBC her son was on his first holiday abroad without parents.

“He’s just an all-round nice, bubbly guy with hundreds of friends who love being in his company,” she said.

“He’s gorgeous, he’s beautiful. He’s my baby.”

Mr Hargreaves’ mother Rachel Hargreaves said she and her son were staying in Tenerife with Ms Duncan for “as long as it takes”.

She said: “You can’t put into words how you feel. I’m just here trying to support and be strong for her.

“The things she can’t do I am trying to do for her, mother-to-mother.”