From the Wikipedia page [1]

Titulus Regius ("royal title" in Latin) is a statute of the Parliament of England, issued in 1484, by which the title of King of England was given to Richard III.

It is an official declaration that describes why the Parliament had found (the year before) that the marriage of Edward IV to Elizabeth Woodville had been invalid, and consequently their children (including their sons Edward V and Richard of Shrewsbury, 1st Duke of York, as well as their eldest daughter Elizabeth) were illegitimate (and, therefore, debarred from the throne), and why Richard III was proclaimed the rightful king.

The act was repealed by the first parliament of the new king, Henry VII. Henry VII also ordered his subjects to destroy all copies of it (and all related documents) without reading them. So well were his orders carried out that only one copy of the law has ever been found. A copy of the law was transcribed by a monastic chronicler into the Croyland Chronicle, where it was discovered by Sir George Buck more than a century later during the reign of James I of England.

The text of the repealing Act has not survived in the records of Parliament, but a contemporary law report (Year Book 1 Henry VII, Hil., plea 1) reproduces part of the text as follows:

...that the said Bill, Act and Record, be anulled and utterly destroyed, and that it be ordained by the same Authority, that the same Act and Record be taken out of the Roll of Parliament, and be cancelled and brent ['burned'], and be put in perpetual oblivion.

The 100-year gap during which Titulus Regius was censored coincided with the ruling period of the Tudor Dynasty. During this period, Richard III was depicted as a usurper by writers such as William Shakespeare and Thomas More. The Tudor King Edward VI of England was enumerated to recognise the legality of Edward V's reign.

The original text of Titulus Regius can be seen here [2].