Actor Chris Pratt has urged his Twitter followers to pray or a 4-year-old fan who is battling brain cancer.

The Jurassic World star told the 3.58 Twitter users that follow him: "The power of prayer saved my son. This boy Sam needs a miracle. Say a prayer for Sam. Even if you never have b4. Try."

His tweet was prompted by a request from a fellow Twitter user Spencer Hodnett, Sam's father. He messaged Pratt asking for him to encourage his millions of fans to pray for the sick child.

"Would love a RT asking followers to say a prayer for a 4 yr old fan. Psalm 107:28-30; MT 21:21"

Pratt's tweet was retweeted more than 2,000 times and he liked by more than 11,000 people.

It received a fair amount of comments too. Some users said they were "unfollowing" the Guardians of the Galaxy actor's account because of the content of his tweets, but many came to his defence, supported his comments and pledged their prayers for young Sam.

One user replied to Pratt's comment with "How so?" to which the father of one responded with another tweet. Although this time he typed out a short prayer: "Lord I pray for Sam. May you put your healing hand on him father. May his family find grace in this difficult time."

This second tweet was retweeted almost 900 times and liked by almost 8,000 Twitter users.

Hodnett later expressed his gratitude for the support by tweeting: "For all the prayers; thank you from the bottom of my heart. Also, my son has the finest team of physicians and therapists in the world."

Jack, Pratt's 3-year-old son with wife actor Anna Faris, was born two months premature.

Pratt often shares pictures of his son on social media and is known to caption his photos on Instagram with Bible verses or references to God.  He has previously spoken about his son's early arrival and subsequent health complications, as has Faris.

Us magazine reported on Pratt's speech about his son's birth and the moments that followed.  Speaking at the March of Dimes' Celebration of Babies Hollywood Luncheon in 2014, Pratt said: "He was hooked up to these monitors and to us it meant that the machine started beeping because he stopped breathing over and over...One of the brave and amazing nurses would come over and shake him a little bit, I guess to remind him he was alive. The only people allowed to touch him were his mom and me, unless you were wearing gloves.

"We did this amazing thing they call 'skin-to-skin' — it was just the best feeling I ever had. It's just what it sounds like. My little boy was laying across my neck and chest feeling my heartbeat and feeling my love, and I played him country music and I sang to him and I made him promises... in that moment... you know, about... just about what kind of dad I wanted to be, and I just prayed that he'd be here long enough and he was going to let me keep him."

Faris told Redbook magazine last year that the whole experience had brought the couple closer.