The Archbishop of Canterbury has spoken of his regret over the way homosexuals were treated by the Church in the past but has clarified in the House of Lords his continued opposition to legalising gay marriage.

Speaking in the House of Lords today, the Most Reverend Justin Welby said it was "essential" that faithful same-sex relationships be "supported with as much dignity and the same legal effect as marriage".

He said it was "absolutely true" that the Church had "often not served the LGBT communities in the way it should".

"I must express my sadness and sorrow for that considerable failure," he said.

However, the Archbishop went on to reiterate that he could not support legalising gay marriage. He said "considerable hesitations" remained over the content of the Marriage (Same-sex Couples) Bill.

"It confuses marriage and weddings. It assumes that the rightful desire for equality – to which I've referred supportively – must mean uniformity, failing to understand that two things may be equal but different. And as a result it does not do what it sets out to do," he said.

The consequence of passing the bill would be to diminish the institution of marriage and weaken family, the Archbishop continued.

"The new marriage of the Bill is an awkward shape with same gender and different gender categories scrunched into it, neither fitting well," he said.

"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 – as we've already heard – is weakened."

Peers are due to vote on the bill on Tuesday.