Relatives of the 29 people who died are expected to join members of the local community and public figures at Southend Parish Church of Scotland to honour their memories.
The helicopter was on its way from RAF Aldergrove near Belfast to Inverness on 2nd June 1994 when it crashed into a remote hillside in a ball of flames.
Rev Stephen Fulcher, minister of Saddell and Carradale is leading both services, the first at 11.45am.
He said: "This is the 25th anniversary of a tragedy that had a huge impact on this very small rural community and remains to this day a living memory for many people.
"A lot of local residents have kept up contact with relatives of the people who were aboard the helicopter and continue to hold them in their thoughts and prayers to this day.
"This is an anniversary of national significance and I expect quite a lot of local people will attend the service because many of them vividly remember that fateful day like it was yesterday."
Former local lighthouse keeper Hector Lamont, who was first on the scene of the crash which occurred during thick fog, is expected to be among people paying their respects.
Representatives from the emergency services, Lord Lieutenant for Argyll and Bute, Patrick Stewart, Deputy Provost of Argyll and Bute, Roderick McCuish, and local MP, Brendan O'Hara, will be in attendance.
Following a community lunch in the village hall, a second service will be held at the memorial cairn near the crash site at 3pm.
Rev Roddy McNidder, who was the minister at Southend at the time of the disaster and helped to support the victim's families, will deliver a sermon.
The minister, who is still in touch with relatives to this day, said the 25 passengers - some of whom were members of the intelligence community - and four crew will never be forgotten.
Mr McNidder will say: "This lovingly built cairn claims the ground in remembrance of the sadness of June 2, 1994, to honour your loved ones and also yourselves, your families and friends.
"Each name inscribed on this cairn, which shines out every time the sun reflects upon it, calls us to remember the unique person whose name is written there.
"And to acknowledge our memories, experiences, and encounters with them, along with the hopes of what might have been over these 25 years and years still to come.
"For memories engraved within our hearts will never be forgotten
"The love shown to us will never be lost."
Last week, the General Assembly of the Church of Scotland issued a fresh call on the Ministry of Defence to keep all records relating to the crash in a "safe place and not deleted".
There are fears that the true cause of the accident, the worst RAF disaster in peacetime, will never be known if this happens.
Former Moderator Very Rev Dr Alan McDonald told commissioners that the Ministry of Defence had confirmed that records closed in 1995 and 1996 "will be reviewed for release or alternative disposal this year".
The pilots, Flt Lts Jonathan Tapper and Richard Cook, were accused of gross negligence.
In 2003, the General Assembly called on the Ministry of Defence to "revisit" the tragedy while Jonathan Tapper's father, Michael, watched from the public gallery.
A fresh review was ordered and in 2011 found that the pilots should not have been blamed and the earlier ruling was set aside.
But David Hill, a retired MOD helicopter engineer and Dr Susan Phoenix, whose husband RUC Det Supt Ian Phoenix was killed, recently said the review had no remit to inquire into the actual cause of the crash.
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