The Queen tells of her joy after couple end suspense

The couple will marry in London in the spring or summer of 2011 after the Prince proposed to his long-term girlfriend using the engagement ring of his late mother, Diana, Princess of Wales.

They got engaged while on holiday in Kenya last month, nine years after they met as art history students at St Andrews University.

William revealed he had kept the ring in a rucksack for days before but refused to disclose whether he went down on one knee to propose, saying: "That's going to stay a secret."

He said the couple had been talking about marriage for more than a year, but now was "just the right time" to become engaged.

"I had my military career and I really wanted to concentrate on my flying," he added, "and I couldn't have done this if I was still doing my training so I've got that out of the way and Kate's, you know, in a good place in terms of work and where she wants to be and stuff and we both decided now was a really good time."

The Prince revealed the couple intended to have children, but wanted to take things "one step at a time".

"We'll sort of get over the marriage first and then maybe look at the kids," he said. "But obviously we want a family so we'll have to start thinking about that."

The couple split temporarily three years ago, a break-up which they admitted had made them stronger. William denied that his military life had driven them apart.

Miss Middleton said: "I at the time wasn't very happy about it, but actually it made me a stronger person, you find out things about yourself that maybe you hadn't realised,

"I think you can get quite consumed by a relationship when you are younger and I really valued that time for me although I didn't think it at the time."

The news was welcomed by other members of the Royal Family, although the Queen and the Prince of Wales both remarked on William's delay in proposing.

The Queen told a guest at a Windsor Castle reception: "It is brilliant news. It has taken them a very long time."

Charles said he was thrilled about his son's engagement, joking: "They have been practising long enough."

Prince Harry said he was "delighted", welcoming Prince William's fiancee Kate Middleton as the "sister" he had always wanted.

Diana's brother, Earl Spencer, said: "It's wonderful news. Very exciting."

Speaking outside their home in Berkshire, Miss Middleton's parents Michael and Carole said they were "absolutely delighted".