The historic document drawn up by King Charles II to outline the terms on which he would return to Britain and assume the throne is expected to fetch up to £600,000 at auction.

Sotheby’s in London will offer one of just two surviving copies of the Declaration of Breda, which is signed by Charles II, as the star lot of its “Coronation Sale” auction on 4 May.

The historic document re-established the British monarchy following the execution of King Charles I in 1649 and the death of Oliver Cromwell, who had served as Lord Protector during the crown’s absence, in 1658.

Charles II drew up the declaration in 1660 during a time of public unrest in Britain.

The document would prove to be his ticket out of exile and back to London where he would rule for the next 25 years.

His father was executed after the English Civil War, between the royalist supporters of the monarchy and Cromwell’s parliamentarians.

The Declaration of Breda outlined how the monarchy would once again rule the country, with the terms still applying today as King Charles III prepares for his coronation on 6 May.

The document is expected to fetch between £400,000 and £600,000 when it goes up for auction two days before the coronation.

Gabriel Heaton, Sotheby’s manuscripts specialist, said: “Alongside the Magna Carta, the Bill of Rights and the Act of Settlement, this is one of a small number of transformational royal documents that have changed royal power forever, and as such it is the most important of its kind to ever appear for public sale.”

Five copies of the document were prepared by Charles II from exile in Breda (in the Protestant Netherlands) in April 1660.

Each was to be sent to the principal power bases in Britain: the House of Commons, the City of London, the Army (these three copies are now presumed lost), the House of Lords (copy now in the parliamentary archives), and the Navy.

It is the copy sent to the Navy which is to be offered for sale at Sotheby’s, and which first passed into the hands of one of the most famous names of the era, Samuel Pepys, who was secretary to the general at sea, Sir Edward Montagu.

The document is entering the public domain for the first time in 40 years since its auction in 1985 from the collection of the descendants of Montagu, who was the man responsible for bringing the king back from exile in the Netherlands in 1660.

One of the most dramatic changes in British constitutional history had been accomplished in a matter of weeks and without significant bloodshed.

The declaration strikes a powerful tone of reconciliation with its hope that the restoration of the king would bind the wounds of civil war, its expression of compassion for all his subjects, and its promise to not seek revenge for past wrongs.

It reads: “To all Our loving Subjects… after this long silence, We have thought it Our Duty to Declare how much
We desire to contribute thereunto… We can never give over the hope in good time to obteyne the possession of that Right, which God and Nature hath made Our Due… after so long misery & sufferings… with as little bloud and dammage to our People, as is possible…”

The Declaration of Breda will be on display in Sotheby’s New Bond Street galleries alongside further coronation sale highlights from 21 April to 4 May.