Campaign group Republic has called for independent authorities to audit and disclose royal tax and income.

The call comes as the monarch prepares to declare what tax he pays on his income. Republic has pointed out that this leaves many unanswered questions.

The move is likely in response to collapsing support, with an Ipsos poll out on the weekend showing the royals on an all-time low.

Speaking for Republic, Graham Smith said today:

"The royals can't be allowed to self-declare their tax. They'll spin this as Charles being a huge tax payer, but the question is why is his income so high?"

"Given royal access and influence the public deserve full disclosure on income, investments and taxes. We need a genuinely independent review of their income, expenditure and taxation."

"And questions remain about why income tax is voluntary, what they off-set against tax, and why Charles was able to avoid a multi-million pound inheritance tax when his mother died."

"William also has questions to answer. Why are we effectively paying William and Charles a personal income of more than £20m each from the two Duchies. These are public assets."

"The main issue remains however: why the monarchy costs the country well over £500m a year, far more than they admit to."

The details of Duchy ownership and the full half a billion pound cost of the monarchy can be found in two reports:

Full cost: https://www.republic.org.uk/halfbillionroyals

Duchies: https://www.republic.org.uk/duchies

Republic is due to release a further report this week on the future of the Sovereign Grant and is working on a response to the recent National Audit Office report on royal residences.