Members of the Ahmadi community will today inaugurate what they describe as the largest mosque complex in western Europe, able to accommodate more than 10,000 worshippers.
The Baitul Futuh Mosque in Morden, Surrey, which features a 50ft stainless steel dome and a 100ft minaret, opens amid controversy.
The community is not recognised by other Muslims as genuinely Islamic.
The building, a few miles from the community's first London centre in Putney, south-west London, built nearly 80 years ago, is expected to be packed with devotees from around the world for the inaugural prayers to be held by the worldwide supreme head of the community, Hadhrat Mirza Masroor Ahmad.
But Iqbal Sacranie, the secretary general of the Muslim Council of Britain, said the community had exaggerated its numbers and influence.
He argued that it should not even use the word mosque to describe the building. "They can call their place of worship by any name except for a mosque because that is for Muslims," he said. "They are outside the fold of Islam."
A person could be a Muslim only if he or she believed that the prophet Mohammed was the last and final messenger of God, he said.
However, the community, which was founded by Hadhrat Mirza Ghulam Ahmad in northern India in 1889, believes that its founder was the final messanger. It claims to have 200 million followers.
The building, which took two years to complete, cost about £15 million, most of it raised from donations by devotees.
Rafiq Ahmed Hayat, the community's British president, said: "Islam has come under a lot of criticism. One of the things we want to do is redress this imbalance.
"The basic tenet of Islam is that of peace and the tenet of this mosque is to propagate that tenet throughout the world.
"We will release some white doves to symbolise peace."