Britain on Thursday withdrew the Syrian ambassador's invitation on the eve of the royal wedding and it emerged that Kate Middleton will not promise to obey Prince William in her marriage vows.

Seeking to end a diplomatic spat that was fast becoming a headache for organisers, Foreign Secretary William Hague said it was ‘unacceptable’ for the envoy to attend because of Syria's crackdown on pro-democracy protesters.

"In the light of this week's attacks against civilians by the Syrian security forces, which we have condemned, the Foreign Secretary has decided that the presence of the Syrian ambassador at the royal wedding would be unacceptable and that he should not attend," a Foreign Office statement said.

Buckingham Palace, the official residence of Queen Elizabeth II, Prince William's grandmother, said it agreed with the move.

The decision that ambassador Sami Khiyami will be barred from joining dozens of envoys from countries around the world at Friday's ceremony followed criticism from rights groups.

The Foreign Office had defended the initial invitation, saying the Syrian envoy had been invited alongside all countries with which Britain has normal diplomatic relations and which have ambassadors in London.

Bahrain's crown prince has already turned down an invitation following fears his attendance could have detracted from the event because of ongoing unrest in the Gulf kingdom.

It is the latest in a string of political controversies surrounding invitations to the wedding, Britain's biggest royal celebration in 30 years and a major chance for the country to strut the world stage.

Preparations reached fever pitch as Kate attended a second and final rehearsal at Westminster Abbey on Thursday morning, accompanied by William's brother and best man Prince Harry.

The Order of Service was released, showing that Kate will pledge to ‘love, comfort, honour and keep’ William rather than the more traditional vow to ‘obey’ him.

It means the 29-year-old is following in the footsteps of William's late mother Princess Diana, who later divorced his father Prince Charles.

Diana, who died in a car crash in Paris in 1997, also opted not to ‘obey’ the Prince of Wales during their wedding ceremony in 1981.

In a message in the souvenir programme, Kate and William, the 28-year-old second-in-line to the throne, say they are deeply touched by people's reactions as they prepare for ‘one of the happiest days of our lives’.

"We are both so delighted that you are able to join us in celebrating what we hope will be one of the happiest days of our lives," they say in the booklet which will be sold along the processional route of the wedding.

Music played at Diana's 1981 wedding to Prince Charles and her funeral in 1997 will be included in the service, the official schedule showed. It will also feature music from Charles's second marriage to Camilla, Duchess of Cornwall, in 2005.

William has spent the days before the wedding in relaxed mood, even joining his friends for a game of football in a London park on Tuesday, newspaper photographs showed.

The prince left the match at Battersea Park on a Ducati motorbike, still wearing a tracksuit and trainers.

A tented village is growing in front of the Abbey as royal enthusiasts continue to pitch tents to get a good position to see the service.

But forecasters said it was now certain that after days of sunshine in London, Friday would be marred by rain showers and even thunder.

In Bucklebury, Kate's home village in southeast England, organisers were erecting tents and preparing for a giant party on Friday.