The parents of missing backpacker Gareth Koch have sent a third letter to the Prime Minister Tony Blair asking him for help in the search for their son who has been missing in Nepal for eight months.

David and Gillian Koch first wrote to Mr Blair in July, asking him to use his influence to ensure the search for their 25-year-old son was carried out thoroughly.

But they have yet to receive a reply to their appeal or an acknowledgement of their letters.

An attempt will also be made to arrange a meeting with Foreign Secretary Jack Straw to discuss the family's plight.

The Conservative prospective parliamentary candidate for Wantage, Ed Vaizey, said he would request a meeting with Mr Straw through the town's retiring Tory MP Robert Jackson.

"We cannot guarantee a meeting but it is clear the Koch family have concerns about the investigation into their son's disappearance and Mr Straw should be made aware of them," he said. Mr Koch, of Hamfield, Wantage, said: "We have been promised that the Prime Minister's office would respond, but we have not received a reply so we have written again hoping that third time is lucky for us."

He added in his letter: "I have to say that I am extremely disappointed that neither you nor any of your staff has taken the time to write or phone in acknowledgement to my original letters.

"My wife Gill and I have just returned from Nepal where we spent four weeks searching for our son.

"We were extremely disappointed and frustrated in the way that we were treated by the Nepalese authorities. Our only hope was that the British Embassy in Nepal would be able to look after our interests, but they have said that they have no money for 'this type of thing'."

A spokesman for the Prime Minister's Office said the letters had all been passed to the Foreign Office to deal with.

A Foreign Office spokesman said it had been supporting the couple since their son went missing, including organising a press conference in Kathmandu to appeal for information about his disappearance.