The famous London church will be used on May 3 by the Royal Navy.
The service is expected to include prayers for peace around the word.
Russell Whiting from the Christian Campaign for Nuclear Disarmament has questioned why the Abbey would want to host an event like this.
Speaking to Premier, he said: "During the Sermon on the Mount, Jesus said, 'Blessed are the peacemakers they will be called the children of God'.
"I don't believe that we can call ourselves peacemakers whilst we possess a weapon that can destroy creation, and it is a creation after all, that we have been given to be stewards of."
While some have labelled it a "celebration" or a "thanksgiving service", Westminster Abbey has distanced itself from those particular words.
In a statement to Premier, a spokesman said: "The service marking the 50 years of the continuous at sea deterrent is not a service of thanksgiving or a celebration of nuclear armaments.
"The service will recognise the commitment of the Royal Navy to effective peace-keeping through the deterrent over the past fifty years and will pray for peace throughout the world."
Russell Whiting told Premier he remains unconvinced.
"The Royal Navy's press release announcing this service described it as a celebration," he said. "[The Abbey] says it's not going to be a thanksgiving but the invitations that have gone out describe this as a national service of thanksgiving.
"It may well be that Westminster Abbey has one thing in mind, but it's clear that the Ministry of Defence has something quite different."
The Christian Campaign for Nuclear Disarmament says it will protest at the event if it goes ahead.
Whiting said he doesn't object to a service going ahead as long as it isn't in a place of Christian worship.
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