A new concession relating on admitting partners in common law and same- sex relationships will take effect from Monday. But Mr O'Brien said the criteria applied to them would be "much tighter than for those who can marry", thus retaining the "special position of marriage".
Under the new rules, a couple will have to show that they have been living together for four years or more and intend to continue to live together permanently. Once admitted, they will have to show that the relationship has continued for a further year before being allowed to settle in the United kingdom.
In a statement, Mr O'Brien said Labour had always supported the fundamental principle of immigration rules, that someone already settled in the UK - such as a British citizen - could bring their spouse to join them, provided they met tests on the genuineness of the marriage and financial self-sufficiency.
To ensure good administration of the immigration system, "legal marriage must be the key to entry for the partner who is not British or not settled here", said Mr O'Brien. However, some couples were barred by law from marrying, either because one partner could not remarry or because they were of the same sex.
The new proposals were criticised by former Tory Home Office minister Ann Widdecombe, who said: "It undermines marriage and secondly it undermines immigration control. So the Labour Party have managed to deal a severe blow at both in one fell swoop."Reuse content