Both sides have now adopted the idea of a royal charter - a formal document used to set up bodies such as universities and the BBC - but there are key differences between the two plans.

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Both the prime minister and the two other parties agree on the concept of a royal charter, a formal document used to establish and lay out the terms of an organisation, for example the Bank of England or the BBC, that cannot be changed without government approval.

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Royal Mail said it '"did not recognise" the document and said it did "not reflect Royal Mail's policy, strategy or position in relation to the current dispute with the CWU".

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Dated 24 September 2009 and marked "In strictest confidence", the document says that "through the dispute" the Royal Mail will "deliver the necessary 2009 changes with or without union engagement".

BBC: CWU chief warns of extra strikes

Previously, a document leaked to BBC's Newsnight programme appeared to show Royal Mail digging in for a bitter battle against the union.

BBC: CWU chief warns of extra strikes

As for the Royal Bank bid, NatWest said it would respond to that in greater detail once the Scottish bank released its offer document later this week.

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