The mother of a missing charity worker has urged Prime Minister David Cameron to put pressure on the Malaysian government to help find him.
Gareth Huntley, from Cricklewood, north London, disappeared on Tuesday after embarking on a trek in Tioman Island.
His mother, Janet Southwell, from Bradford, has written an open letter to Mr Cameron urging him to speak to the Malaysian leader.
Downing Street said the government was working hard with the Malaysians.
A Malaysian police team has arrived on the jungle-covered island to search for Mr Huntley, who had been working at the Juara Turtle Project.
A team of up to 30 people and one dog arrived on Saturday to try and find him, locals have told his family.
'Out of character'
Ms Southwell is due to fly out to Malaysia later and hopes to arrive by Monday.
In her letter to Mr Cameron, she wrote that she was "humbly asking" him to intervene.
"Please do the right thing prime minister and make one phone call to the Malaysian leader to insist that they deploy real help to find Gareth before time runs out," she said.
"He is out there in the jungle and he needs help."
A Downing Street spokesman said it was working to ensure a "comprehensive search is carried out".
"The Foreign Office continue to work with the Malaysian authorities," the spokesman added.
"An FCO consular team is en route to Tioman Island to meet the family when they arrive."
Mr Huntley's brother Mark, who lives in Leeds, told BBC 5 live's Stephen Nolan programme that for days it had only been local people and Gareth's friends who were searching for him.
He added he wanted British authorities to "put more pressure on the Malaysian police force and search and rescue to do more to intensify the search".
He said his 34-year-old brother's disappearance was out of character and he would normally be good at keeping in touch.
"For him to not turn up and still be missing at this point - he is not like that, he would have made contact by now," he added.
'No-one in control'
The family fears Mr Huntley either broke his leg and was unable to get help or was bitten by a snake.
His mother said he should have been back from his trek within two hours and he was carrying water and wearing proper footwear.
A friend of Mr Huntley, Kyle Neo Kai Fu, who lives locally, said: "The idea that there is a proper professional search going on is laughable. There are five police, in one boat, and nobody is in control.
"The most likely scenario is that Gareth is injured in a ravine, with a broken leg - and we are running out of time."
Mr Huntley's friend Hannah Alice Grant, who is on the island, has written a detailed account on Facebook of the search for him.
She wrote that when she did the walk to the waterfall, it was very easy to slip over or fall.
The campaign group trying to find him said it was sad that someone had set up a fake fundraising account and that people should not donate to it.