The 84-year-old's estranged son reported that his father, once a pastor of the church notorious for its homophobic and racist views, was in hospice care and near death.
In a Facebook post Nate Phelps said his father "is now on the edge of death at Midland Hospice house in Topeka, Kansas."
He said he had also learned that Phelps Sr. was ex-communicated from his own church in August of 2013.
"I’m not sure how I feel about this. Terribly ironic that his devotion to his god ends this way. Destroyed by the monster he made."
A second estranged Phelps son, Mark, told Kansas newspaper The Topeka Capital-Journal the news was accurate.
Meanwhile church spokesman Steve Drain confirmed that Rev Phelps was being looked after in a care facility.
Mr Drain said: "I can tell you that Fred Phelps is having some health problems."
He did not elaborate on what those problems were.
The Church won the right to stage high-profile protestsin 2011 Phelps Sr, once an award-winning civil rights attorney, founded the church in Kansas in 1955.
It has shot to notoriety in recent years after its members staged numerous protests at the funerals of dead US soldiers.
Images of its members carrying placards reading "Thank God for dead soldiers" and "Thank God for 9/11" earned the church significant attention around world, as did its claim that the deaths were God's punishment for immorality and tolerance of homosexuality and abortion.
Anger over the behaviour of Westboro members, largely made up of Phelps' extended family, inspired a federal law and laws in numerous states limiting picketing at funerals.
But, in a landmark free speech ruling in 2011, the US Supreme Court found that the church had the right to stage protests.
It ruled it could not be sued for monetary damages for upsetting grieving families.
The story of the Westboro Baptist Church has inspired numerous documentaries, including British journalist Louis Theroux's The Most Hated Family In America.
(function(d){ var js, ref = d.getElementsByTagName('script')[0]; js = d.createElement('script'); js.id = 'outbrainjs'; js.async = true; js.src = "//widgets.outbrain.com/outbrain.js"; ref.parentNode.insertBefore(js, ref); }(document));
The buildings housed 15 apartments, a ground-floor church and a piano shop
Carmen Tanco (L) and Griselde Comacho died in the blast
Domenico Rancadore's home in Uxbridge
Mr Rancadore's wife Anne Skinner (left) and daughter Daniela
The Government has chosen Ebbsfleet in Kent as a new garden city
Details of a planned £1,200 childcare tax break will be unveiled
Ed Balls says Mr Osborne's housing measures 'fall short'
Owner Assem Allam has threatened to walk away from the club
The Hinduja brothers are worth £6bn from trucking and banking
Newcastle United owner Mike Ashley is valued at £3.3bn