Shaw's love for real-life Eliza Doolittle

She was the inspiration for George Bernard Shaw's most celebrated creation, and the woman who first immortalised Eliza Doolittle on stage.

Now the true extent of Shaw's feelings for Edwardian actress Mrs Patrick Campbell can be revealed through an extraordinary collection of personal documents to be auctioned by her great granddaughter.

The "platonic love affair" between the playwright and his muse has never been a secret in literary circles. Their mentor-pupil relationship found its clearest expression in his most famous play, Pygmalion, which he tricked her into reading in the hope she would recognise aspects of herself in the character of its Cockney flower girl heroine.

But until now, few have appreciated the full lengths to which Shaw was prepared to go to prove his undying fealty to Mrs Campbell.

Letters included in this week's sale reveal that when the actress died in poverty in 1940 after emigrating to the south of France, it was Shaw who paid for her funeral.

And far from expecting her family to repay him, as they sought to for decades afterwards, he secretly duped them by burning the cheques they sent him.

In a letter to Mrs Campbell's grandson Pat Beech, dated 9 December 1943, on receipt of one such cheque, Shaw confesses: "This payment puts me in a hole. The truth is I tricked your mother over those cheques which she so scrupulously sent me on account of the £100 (funeral cost) ... I sent her formal receipts, thanked her, and burnt the cheques. She never noticed that they had not been presented. I knew she would not, having played that trick before."

Three months later, in a letter intended to reassure Mrs Campbell's daughter, Stella, over her apparent concern that her payments have never been cashed, he writes: "You must not feel distressed about what you call your debt: you have paid it honourably to the uttermost farthing.

"But I have my rights and duties in the matter too. I was her best friend: you were only her daughter, a whole generation removed.

"It was for me, not for you, to render first aid. So everything is as it should be."

Further evidence of Shaw's attachment to Mrs Campbell can be glimpsed in other papers in the sale, including an earlier letter to Stella, also an actress, in which he gives her a pep talk about how to succeed in her profession.

The letter, which at one point refers to Mrs Campbell as someone with "the constitution of Colleoni's bronze charger in Venice", advises Stella: "... You shouldn't smoke; and you shouldn't fast and pray, and make practice of going up and down stairs on your hands with your boots in the air and so forth..."

The collection also contains various other mementoes, including a poem penned by Shaw for Mrs Campbell, entitled "Stellinetta Sings to her Banjo". The humorous verse, dated August 1912, opens with the stanza:

"He's mad! mad! mad!

He's gone right off his chump

He cleans his boots with strawberry jam;

He thinks the world of my silly old mam

Who doesn't value his plays a dam

For they give her the blooming hump, dear girls;

They give her the blooming hump."

The same lot includes an intriguing set of papers that give an insight into Mrs Campbell's wider appeal among the literary set of her day. One is the first draft of an early poem by W B Yeats, "At Galway Races", believed to have been written with Mrs Campbell and her daughter in mind. The poet is said to have been bewitched by Stella when he met her while her mother was appearing in his play Deirdre at Dublin's Abbey Theatre.

Another lot offers a further glimpse of the extent to which Yeats became besotted with Stella, in the form of a letter concerning an astrological chart he has prepared for her.

In it, he gushes: "You have a most emotional artistic temperament and I think I would have known from the stars that you have acting power."

The letters will be put up for sale by Jennifer Forbes, the great-granddaughter of Mrs Patrick Campbell, who was born Beatrice Stella Tanner, on Wednesday at Dominic Winter Book Auctions in Swindon.