Tony Blair and Gordon Brown have not received invitations to the royal wedding - unlike Sir John Major and Margaret Thatcher.
News of the decision to exclude them from the Westminster Abbey ceremony prompted claims from some Labour MPs that the pair had been unfairly snubbed by the monarchy.
When Prince Charles married Diana Spencer in 1981 at St Paul's Cathedral all surviving former prime ministers were invited.
Sir John Major will attend but Lady Thatcher has declined on health grounds. A royal spokesperson said the Conservative ex-PMs had been invited because they were knights of the garter.
Sir John was invited because he was made a guardian of Princes William and Harry after Princess Diana's death in 1997.
"It is a private wedding and the couple are entitled to invite whoever they want," the spokesperson told the Herald newspaper.
"Prince William is not the Prince of Wales or the King, and he hasn't got that link to prime ministers in the way that the Queen does."
The 1,900-strong congregation will be graced by the presence of a galaxy of celebrities, sports stars and foreign dignitaries.
But Mr Blair and Mr Brown, who occupied No 10 for 13 out of the last 14 years, will not be among them.
"Those who have been prime minister have served this country, and I think that the same proprieties that have been followed on previous occasions should have been followed again," Labour frontbencher Chris Bryant told the Mail newspaper.
"I blame Downing Street for not spotting it and saying 'we don't like Gordon Brown either, but he really should be there'."