As a penniless young man, Albert Gubay made a promise - if God helped him become a millionaire, He could have half the money.
Decades later, he has gone even better, donating almost his entire wealth to charity.
Thanks to the success of Kwik Save supermarkets and the Total Fitness gym chain, Mr Gubay - a Roman Catholic - is worth almost £500million.
And at the age of 82, he is determined to double that in his lifetime before turning
over an annual income estimated at £20million to good causes - half connected to the Catholic Church.
He has already put £480million into a charitable trust, leaving him a 'mere' £10million to get by on.
Last night Mr Gubay said: 'I want to carry on supporting good causes, but my whole focus in the next few years is to work as hard as I can to meet my target of a £1billion charity. Every penny wasted or lost reduces the pot available to the charity.'
Born in North Wales to a Jewish Iraqi refugee father and an Irish Catholic mother, the young Mr Gubay cut his entrepreneurial teeth selling sweets.
He launched the first Kwik Save discount store in Prestatyn in 1965 and sold the chain eight years later for £14million.
The proceeds were invested in property, and he became a familiar presence on his developments - even helping out as a labourer, earning him the nickname of 'Britain's richest navvy'.
Mr Gubay settled in a luxury estate on the Isle of Man, and later, while recovering from a back injury, set up what became the Total Fitness network of gyms - he sold this in 2004 for £70million.
In a 1997 television documentary, he said: 'After the war I came out of the Royal Navy with a demob suit and £80.
'I borrowed £100 and made the pact with God: make me a millionaire - and you can have half of my money.'
He added: 'My belief in a day of reckoning keeps me on the straight and narrow.'
Mr Gubay, who lives with his second wife, Carmel, will continue running his companies until he dies, maximising profits for the Albert Gubay Charitable Foundation-which now owns them.
Half the income must be spent on projects connected with the Catholic Church with the rest distributed at the discretion of the trustees, chaired by John Nugent.
Mr Nugent said yesterday: 'Albert is a very frugal man and has dedicated his life to good causes.
'His priority now is to maximise the asset base of the company.'
Mr Gubay may not be Britain's biggest- ever charitable donor, however. Former science minister Lord Sainsbury last year gave £1billion to charity.