Republic, the organisation campaigning for an elected head of state, has called on official tourism body VisitBritain to come clean over its claims that the royal wedding will be a boost for the tourist trade.
Internal emails obtained by Republic under Freedom of Information rules show VisitBritain has little evidence to support their claims, while their own research suggests tourism may suffer during the wedding period.
Republic spokesperson Graham Smith said: "We will be writing to ministers and the chair of VisitBritain's board demanding a review of the organisation's recent behaviour in promoting the monarchy."
"The palace has made a habit of trying to excuse its extravagant costs by claiming credit for tourism revenue, it's very worrying to see that VisitBritain has allowed itself to be coopted by the palace to help perpetuate this myth."
"VisitBritain is supposed to be an independent and credible tourism body, yet it has apparently flatly contradicted its own internal evidence so it can play along with this fiction that the monarchy - and specifically the wedding - is a boost for the tourism industry.
"VisitBritain also makes the unsubstantiated claim that the monarchy brings in £500 million of tourism revenue each year, despite a complete lack of evidence to support this assertion.
"It is the job of VisitBritain to promote Britain as a tourist destination, not to promote the monarchy as an institution. They must promote Britain in accordance with the evidence on what people find attractive and appealing. By allowing themselves to be coopted in this manner, by contradicting their own evidence they have brought into doubt their own integrity as an organisation."
He added "I have seen no evidence tourism would suffer if the monarchy were abolished, on the contrary it's easy to believe that it would marginally improve if all the castles and palaces were open to visitors. Even so, making money selling postcards and hotel rooms is never a good argument for abandoning any hope of real democratic reform."