A mother who lost her son to cancer burst into tears when singer Taylor Swift called to tell her she had written a song in tribute to the late youngster.

Maya Thompson, whose son Ronan passed away last year, wrote about her experiences online and Swift was so moved by the blog she penned a touching track in tribute to the four year old.

Swift debuted the song, simply titled Ronan, during the Stand Up 2 Cancer telethon on Friday night, and Thompson has now opened up about the track.

She reveals she was unaware of Swift's latest writing project until she received a surprise voice mail message from the singer, and when they finally spoke on the telephone Thompson was moved to tears by the star's efforts.

In a post on her blog, which is addressed to her dead son, Thompson writes, "My calmness soon turned to complete and utter frozen shock when these words came out of her mouth. 'I wrote a song for Ronan,' she said.

"The tears started pouring down my cheeks as soon as I heard her say those words. But her words didn't stop there. Not only did she write a song for you, but she wanted to know if it would be alright to perform it on the nationally televised Stand Up 2 Cancer show... She wanted to make me co-author of the song with her. She talked about how... reading this blog and following our story, has inspired her... I don't even remember what I said to Taylor... all I could pretty much do was cry and tell her thank you."

"How do you even find the words to thank somebody for something like this? I was a blubbering mess telling her how much this meant to me... I got off the phone and I was shaking, crying, and could not even form a thought."

 In a later post, Thompson reveals she was overwhelmed with emotion when she watched Swift perform the song for the first time on TV: "Everyone was crying... I couldn't even focus on the words that were coming out of Taylor's mouth. I could see... the raw emotion, sadness, beauty, and pain that filled her face. She looked like an angel."