The discovery of a cache of hitherto unknown letters sheds new light on John Knox, leader of the Scottish Reformation.

Professor Jane Dawson of Edinburgh University, who uncovered the letters exchanged with Knox's best friend Christopher Goodman, has now written a landmark new book called John Knox, launched at St Giles in the Scottish capital on Wednesday.

It is claimed the book will shatter the perception that Knox had no impact outside of Scotland where he remains a significant character.

It will describe how he was a proud member of the European community of Reformed Churches and deeply involved in religious reformation in England, Ireland, France, Switzerland, and the Holy Roman Empire.

Ms Dawson also discovered that Knox was invited to go on a preaching tour of Ireland and wonders about the potential impact on that country if it, like Scotland, had been exposed to the preacher's powerful sermons.

She said: "I have been fortunate to make some exciting new discoveries.

"I discovered the previously-unknown invitation made in 1566-7 to Knox from his best friend Christopher Goodman to join a preaching tour in Ireland.

"Knox might well have accepted but due to rapid changes - not least the fall of Mary, Queen of Scots - it never happened.

"It remains one of those intriguing 'what ifs?'"

The letters contain the first new material from Knox's pen to be found since 1875.

Knox's "table-talk" - a conversation at dinner in his house recorded by his secretary but not identified as Knox's words before now - has also been identified within other memoirs.

John Knox's Bible was discovered in Sydney by the late Professor David Wright, a former colleague of Professor Dawson. Mr Wright's widow passed on the Bible to Ms Dawson to help her with the new book.

A portrait in Edinburgh has been cleaned and re-dated and might prove to be a better likeness of the reformer.

The Church of Scotland said that together the discoveries have uncovered previously-hidden aspects of Knox's life and his character and reveal the private as well as the public face of the man.

Knox is said to have emerged from the book as a complex character not simply as the "personification of the puritanical kill-joy" but describes how he was a man of humour and passion and a person "to whom tears came easily".

The archival discoveries including the cache of papers related to Knox's best friend Goodman is described as brimful of surprises.

While it is no secret that Knox and Mary Queen of Scots were not on friendly terms it is revealed that at one point they worked together in the background after the breakdown in the relationship of an influential earl and countess acting as an "unlikely pair of marriage guidance counsellors".

In 1558 Knox decried what he called a "monstrous regiment" when he published "The first blast of the trumpet against the monstrous regiment of women" in Geneva.

Arguing it was wrong for a woman to rule over a country, his treatise was directed principally against England's Queen Mary, but it did not endear Knox to Mary's sister and successor, Elizabeth I.

When Knox sought to return to Scotland from Switzerland, his journey was delayed as Elizabeth refused him permission to travel through England.

The new book also tells that following Knox's time south of the Border during Edward VI's reign and other times in English communities he "regarded himself as an Englishman by adoption".