Evangelicals are asking for prayers ahead of the Methodist Church's annual conference next month when proposals to allow gay marriage ceremonies will be considered.
The Church's leadership will debate the recommendations from a task group to relax the rules around marriage so that ministers can conduct same-sex wedding ceremonies in their chapels.
The current Methodist Church position on marriage is set out in Standing Order 011A (1), which states: "The Methodist Church believes that marriage is a gift of God and that it is God's intention that a marriage should be a life-long union in body, mind and spirit of one man and one woman."
The report from the Marriage and Relationships Task Group also suggests that Methodist churches be allowed to help cohabiting couples mark milestones in their relationship.
The recommendations have been made in light of dramatic changes to the UK's marriage laws in the last 15 years, with Parliament approving civil partnerships in 2004, followed by gay marriage a decade later. Further changes are afoot as last year, the Government said that heterosexual couples in England and Wales would be able to enter into civil partnerships.
The recommendations from the task group include a clause that would ensure no minister is forced to conduct a same-sex wedding cerermony against their conscience.
Methodist Evangelicals Together has responded with alarm to the task group's report and is calling for people to pray that the Methodist Church remains faithful to the biblical teaching on marriage.
The network said that the Methodist Church in Britain was "at a crossroads" as it comes together to consider its understanding of marriage,
It is asking evangelical Christians worldwide to join in a time of prayer and fasting for the British Methodist Church to "remain faithful to the Bible's teaching".
The suggested period of prayer and fasting is from June 21 until the conference opens on June 27.
In an earlier statement, Methodist Evangelicals Together said: "Methodist Evangelicals Together continues to be committed to the Wesleyan convictions that the gospel is for all, that the Bible is our supreme rule of faith and practice, and that all Christians are called to holiness.
"We therefore uphold the biblical understanding of marriage as the life-long union of one man and one woman, and call upon the Church to do the same.
"We are carefully studying the report, noting that it is intended as a discussion starter rather than reaching definitive conclusions.
The Methodist conference runs until July 4. Methodist Evangelicals Together have published a guide further explaining its position on marriage.