Ahmadinejad, whose comments will go out on 1915 GMT on Christmas Day, will say that if Jesus Christ were alive today, he would oppose "bullying, ill-tempered and expansionist powers," according to a pre-released transcript.
Britain and Iran have had rocky relations since the 1979 Islamic revolution, particularly over Iran's disputed nuclear programme, which the West is concerned could be used to build arms but Tehran insists is for civilian purposes.
Channel Four has invited a figure in the news to deliver an alternative Christmas message to the queen's annual broadcast since 1993, but Ahmadinejad is the most high-profile guest yet.
The message begins with Ahmadinejad congratulating Christians and the people of Britain on the anniversary of the birth of Christ, which Christians celebrate on Christmas Day.
He adds: "If Christ were on earth today, undoubtedly he would stand with the people in opposition to bullying, ill-tempered and expansionist powers.
"If Christ were on earth today, undoubtedly he would hoist the banner of justice and love for humanity to oppose warmongers, occupiers, terrorists and bullies the world over."
Speaking in Farsi, Ahmadinejad says society's problems are down to humanity's rejection of religion but predicts Christ will return "with one of the children of the revered Messenger of Islam and will lead the world to love, brotherhood and justice".
He ends by saying: "I pray for the New Year to be a year of happiness, prosperity, peace and brotherhood for humanity. I wish you every success and happiness."
Explaining the decision to invite Ahmadinejad to speak, Channel Four's head of news and current affairs Dorothy Byrne said: "As the leader of one of the most powerful states in the Middle East, President Ahmadinejad's views are enormously influential.
"As we approach a critical time in international relations, we are offering our viewers an insight into an alternative world view."
In October, Foreign Secretary David Miliband warned of a possible nuclear arms race in the Middle East if Iran was allowed to press ahead unchecked with a uranium enrichment programme.
In response, Tehran accused Miliband of having "strong ties with Zionists."
Ties between the two countries were further strained last year when Iran seized 15 British sailors and marines in disputed waters in the Shatt al-Arab waterway between Iran and Iraq.
The troops were released safely nearly two weeks later after a televised meeting with Ahmadinejad.
Stephen Smith, director of Britain's Holocaust Centre, which aims to educate people about genocide, warned that Ahmadinejad's message should be treated with care.
"People need to be alert to the fact that this is a wolf in sheep's clothing," he said. "Many of his political and historical views are very dangerous and do not uphold the views in his message."
