But Anglican traditionalists criticised the Archbishop, the second most senior cleric in the Church of England, for failing to reinforce Christian teaching which prohibits sex outside marriage.

The row came as Prince William and Kate Middleton unveiled their choices for the royal wedding service, which include classically British music and hymns, and an updated choice of marriage vows in which the bride omits the word "obey".

In a television interview, Dr Sentamu was asked whether it was appropriate for the Prince, who is in line to become head of the Church of England as King, to have been living with his bride before marriage.

He said he had conducted wedding services for "many cohabiting couples" during his time as a vicar in south London.

"We are living at a time where some people, as my daughter used to say, they want to test whether the milk is good before they buy the cow," he said. "For some people that's where their journeys are.

"But what is important, actually, is not to simply look at the past because they are going to be standing in the Abbey taking these wonderful vows: 'for better for worse; for richer for poorer; in sickness and in health; till death us do part.'"

However, the Rev David Phillips, general secretary of the Church Society, a conservative evangelical group, said the Archbishop had "missed an opportunity to set out Christian teaching"

"What he said wasn't appropriate as he," Mr Phillips said. "He gave the impression it doesn't matter whether people live together before marriage. I thought he would have tried to get across Christian teaching on marriage that says it is not appropriate to have sex outside marriage."

In another sign of their modern approach, the Prince and Miss Middleton published their order of service online for millions of people to join in at home.

The 28-page booklet confirms that Kate Middleton will follow the example of Diana, Princess of Wales by omitting the word "obey" from her vows.

Instead she will "love, comfort, honour and keep" him, mirroring the vows the Prince himself will make.

The hour-long ceremony will showcase the best of British music from Tudor times to the present day, and includes new compositions written specially for the occasion, as well as a few surprises.

Miss Middleton will walk down the aisle to I Was Glad by Sir Charles Hubert Hastings Parry, the composer of Jerusalem, which will also feature as one of three hymns chosen by the couple.

Elgar, Vaughan Williams and Delius will feature heavily in the orchestral music played before the service, but the couple have also chosen two pieces by the Master of the Queen's music, Sir Peter Maxwell Davies.

Sir Peter had originally complained of feeling snubbed when the couple did not ask him to compose any new music for their wedding, to the extent that he said he would not watch the service on TV.

When he was told yesterday that his music had been chosen, he said: "It's news to me - nobody told me. But obviously I'm delighted.

"I think they deserve great credit for choosing a British musical feel to their service. It will be a great encouragement to the musical traditions of this country which we have much to be proud of."

Westminster Abbey will give the couple a wedding present in the form of an anthem composed by John Rutter, from London, who has been described as the world's favourite living choral composer.

Meanwhile Paul Mealor, a 35-year-old composer from Anglesey, where the bride and groom live, said he was "thrilled" to discover that his work, Ubi caritas, which premiered at St Andrews University last year, had been chosen for the Motet.

"The ceremony is going to be, without a doubt, the most emotionally intense and exhilarating hour of my life," he said.

A well as Jerusalem, the couple have chosen the traditional hymns Guide Me, O Thou Great Redeemer and Love Divine All Loves Excelling, all of which are "favourites of the couple" according to royal aides.

Three of the pieces being played before the wedding also featured in the Prince of Wales's wedding to the Duchess of Cornwall in 2005, and the Prince is understood to have given the couple a helping hand in choosing the music.

Following the signing of the register, the Fanfare Team of the Central Band of the Royal Air Force will perform a fanfare called Valiant and Brave, after the motto of 22 Sqn, in which Prince William flies as a Search and Rescue helicopter pilot. The fanfare was composed by Wing Commander Duncan Stubbs, the RAF's Principal Director of Music.

The witnesses to the signing will be the Prince of Wales and the Duchess of Cornwall, Prince Harry, Pippa Middleton and Michael and Carole Middleton.

The bride and groom, as man and wife, will walk down the aisle to Crown Imperial, by William Walton.

END