The Archbishop of Canterbury Justin Welby has criticised Britain's same-sex marriage bill, saying it is neither equal nor effective, noting that it will diminish Christian marriage and family life.

"Marriage is abolished, redefined and recreated – being different and unequal for different categories.

"The new marriage of the bill is an awkward shape with same gender and different gender categories scrunched into it - neither fitting well."

In his position as the Archbishop of Canterbury, Welby sits in Britain's upper chamber of Parliament and he spoke in a House of Lord's debate Monday before it was to finish with a vote Tuesday.

The proposed law is backed by Conservative Party leader and Prime Minister David Cameron, his Liberal Democrat deputy Nick Clegg and the opposition Labour Party leader Ed Miliband.

The Marriage (Same Sex Couples) Bill passed through the lower House of Commons in a free vote with a 205 majority after 366 MPs voted in favor, with 161 MPs voting against it on May 21.

Welby, a former banking and oil industry executive who is a family man with five children, became the highest cleric in the Church of England when he became Archbishop of Canterbury in March.

He is also the spiritual leader of the 85-million strong Anglican Communion, which is deeply divided about the issue of same-sex marriage and openly gay clergy.

"The concept of marriage as a normative place for procreation is lost. The idea of marriage as covenant is diminished. The family in its normal sense predating the State and as our base community of society is weakened," said the Anglican leader.

"For these and many other reasons those of us in the churches and faith groups, who are extremely hesitant about the bill in many cases, hold that view because we think that traditional marriage is a cornerstone of society and rather than adding a new and valued institution alongside it for same gender relationships, which I would personally strongly support to strengthen us all, this bill weakens what exists and replaces it with a less good option that is neither equal nor effective."

Welby added, "This is not a faith issue, although we are grateful for the attention that government and the other place have paid to issues of religious freedom - deeply grateful.

"But it is not, at heart, a faith issue; it is about the general social good. And so with much regret but entire conviction, I cannot support the Bill as it stands."