A senior bishop has hit out at plans to allow transsexuals to marry in their adopted sex.
Transsexual Elizabeth Bellinger and her husband Michael
Bishop of Winchester, the Rt Rev Michael Scott-Joynt, said the move would undermine marriage as everyone understands it, between men and women.
The measure is a key part of the Gender Recognition Bill, which is likely to face a rocky ride through Parliament.
But Lord Filkin, for the government, said the bill reflected Labour's commitment to "social inclusion".
Mr Scott-Joynt said "marriage according to the law of the land is the union of one man with one woman and that is proclaimed by the plaque in the wall of every Register Office".
He added: "To change fundamentally that character of marriage will have incalculable effects down all the years to come.
"If this bill is passed into law as it stands, the words 'woman' and 'man' will no longer mean what everyone including the law has always thought that they meant.
"Whatever its protestations, the government will have introduced marriage of two people of the same sex."
The draft Gender Recognition Bill was published one year after a landmark ruling by the European Court of Human Rights that current UK law breached transsexuals' human rights.
Earlier this year transsexual Elizabeth Bellinger and her husband Michael lost their battle to have their 22-year marriage declared legal.
Elizabeth, 56, underwent a male-to-female sex change operation in 1981 and married Michael Bellinger the same year.
The registrar who married the couple did not ask for any evidence of Mrs Bellinger's gender and the couple have lived since then as husband and wife.
But the British legal system as it stands does not accept the validity of the marriage and last year Mrs Bellinger's case was heard by the House of Lords, the highest court in the UK.
In April five Law Lords rejected the Lincolnshire couple's marriage, saying Parliament regards gender at birth as fixed for life.
Officials expect 300 people a year to apply for new birth certificates, once a current backlog of 5,000 is cleared.
Under the proposals a new body, the Gender Recognition Panel, will be set up to assess applications for new birth certificates.
The bill will benefit only a few thousand people, but the government says it is an important measure to restore the human rights of a vulnerable minority.
Transsexuals will have to meet medical criteria, give a sworn oath that they have lived in their new gender for at least a few years and that they intend to continue in their new gender until death.