Here I found an article which gives a bit more information (from AFP, 22.3.2005 - so the info could be outdated).
[FONT=verdana,tahoma,arial,helvetica] However, the Department of Constitutional Affairs confirmed on Monday that Parker Bowles would be queen once Charles takes the throne.[/FONT]
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[FONT=verdana,tahoma,arial,helvetica]"Technically, she will be queen" [/FONT]
[FONT=verdana,tahoma,arial,helvetica] "Technically, she will be queen," department spokesperson Zoe Campbell told AFP. [/FONT]
[FONT=verdana,tahoma,arial,helvetica] The news followed a written House of Commons question to the department by Andrew Mackinlay, a lawmaker for the ruling Labour Party, who asked whether the April 8 marriage would be "morganatic." [/FONT]
[FONT=verdana,tahoma,arial,helvetica] He was using an arcane term referring to cases in which someone of royal or noble birth marries someone of lower rank and does not share any titles. [/FONT]
[FONT=verdana,tahoma,arial,helvetica] Constitutional Affairs Minister Christopher Leslie gave a simple reply: "No". [/FONT]
[FONT=verdana,tahoma,arial,helvetica] "This is absolutely unequivocal that she automatically becomes queen when he becomes king," Mackinlay said on Monday.[/FONT]
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[FONT=verdana,tahoma,arial,helvetica]No formal constitution [/FONT]
[FONT=verdana,tahoma,arial,helvetica] The Department of Constitutional Affairs stressed, however, that much of the argument was academic, given the fact that Britain has no formal constitution, just a series of laws and conventions that have evolved over the centuries. [/FONT]
[FONT=verdana,tahoma,arial,helvetica] There was no single law stating that someone with Parker Bowles's future position "will be known as queen", Campbell said. [/FONT]
[FONT=verdana,tahoma,arial,helvetica] "When you say constitutional and legal norms, a lot of it is down to convention rather than legislation, which is why she can be referred to as whatever she likes without having to change the law," she said. "She can be referred to as the Duchess of Cornwall or Princess Consort without any change to the law." [/FONT]
[FONT=verdana,tahoma,arial,helvetica] There were no moves to change the law to prevent Parker Bowles being queen, Campbell added.[/FONT]
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[FONT=verdana,tahoma,arial,helvetica]Royal convention [/FONT]
[FONT=verdana,tahoma,arial,helvetica] It is royal convention that decrees that the wife of a king is known as queen, while husbands of female monarchs such as Elizabeth's spouse Prince Philip, do not become king. [/FONT]
[FONT=verdana,tahoma,arial,helvetica] This centrality of convention meant that it was simply "not true" to say Parker Bowles would eventually be queen, Prince Charles's office insisted later on Monday. [/FONT]
[FONT=verdana,tahoma,arial,helvetica] "Our position has always been that you become queen only by convention, not by statute, not by law," a spokesperson for Clarence House said. [/FONT]
[FONT=verdana,tahoma,arial,helvetica] "Our understanding is that the government agrees with our position," he said. [/FONT]