The Prince of Wales has reportedly raised concerns about the implications of changing the laws of succession to the British throne.
Charles is said to be in favour in principle of Government plans to end discrimination against female heirs to the throne as long as they are backed by the public.
The measure will mean that the Duke and Duchess of Cambridge's first baby can succeed to the throne, regardless of whether the child is a girl or a boy.
But the Daily Mail reports that in a meeting with Richard Heaton, permanent secretary of the Cabinet Office, the Prince voiced worries about what will happen if his grandchild marries a Roman Catholic.
There are plans to end the bar on royal heirs marrying Catholics if they want to keep their entitlement to the throne.
It is assumed any children born to a future royal and their Catholic partner would be brought up as Catholics. But this could ultimately lead to the constitutional crisis of a future King or Queen being barred from the throne because they are a Catholic - something which is still outlawed.
The Daily Mail reported that Charles was told the problem could be resolved by negotiations with the Vatican but he was said to have found the comment "unsatisfactory and unconvincing".
The Prince was also concerned about the effect the changes to the rules of succession will have for other hereditary titles passed down the male line.
New legislation which will end male primogeniture - the rule that discriminates against royal women - and allow heirs to the throne to marry Catholics was published last month.
The Bill will be considered and voted on in the Commons as soon as parliamentary time allows.