Eighteen charities and organisations will discover this week whether their campaign to radically transform the future of fostering has been successful.
MPs were due to debate on 20 March whether unmarried and single-sex couples should have the right to adopt.
Representatives of the 18 groups, including Barnardo's, the NSPCC, the National Children's Bureau and BAAF Adoption and Fostering, have called for an amendment to the Adoption and Children Bill to allow unmarried couples to adopt.
They argue that with 15 per cent of adults now living together rather than marrying, the Government's aim to increase by 40 per cent the number of children in care adopted would be impossible to achieve.
Under the current law, adoption is only permitted by single people and married couples. The pressure groups argue that the interests of thousands of children would be better served by the amendment.