According to Sky News, Tim Martin has given the cash to Vote Leave campaign following it struggling to keep up with opposition – Britain Stronger in Europe (BSiE).
This makes Martin one of the most recognisable corporate figures to surface as an advocate for either side of the campaigns.
Martin confirmed his hand out on Monday evening and unlike other donors who have chosen to stay anonymous, he has a substantial public profile.
The pub tycoon, who founded JDW nearly 40 years ago, has signed numerous letters saying that leaving will give Britain greater economic freedom.
He has repeatedly said that the bureaucratic red tape restricts small businesses.
The fifth of a million pound donation will be welcomed prior to first disclosure of donations by the Electoral Commissions today (Wednesday).
According to Sky News, the BSiE campaign received £14m which is double what it is allowed to be spent in the ten weeks leading up to polling day.
These donations include million pound sums from hedge fund tycoon David Harding and Lord Sainsbury.
'Insiders' told the broadcaster the amount the BSiE have is more than double than Vote have.
BSiE’s continuous fundraising shows some expenses including staffing costs are excluded from the £7m limit.
According to Sky, a “significant sum has already been spent by the BSiE” before the start of the formal referendum period which began on April 15 this year.
The £14m amount so far may be different to the amount outlined by the Electoral Commission due to the cut of points for regulators donation disclosure.
Publications week will cover February 1 to April 21.
Sky have said that a source close to the Remain Campaign revealed it would go further than the regulatory requirements by naming donors who had given more than £7500 and not just those who had “committed money between February and April”.
The list includes both individuals and investment banks such as Goldman Sachs, Morgan Stanley, Citi and JP Morgan who between them have contributed an estimated £15m, as reported by Sky.
Liberty Global, owned by Liberty General, have disclosed that it was looking for shareholder’s permission to donate up to £500,000 in a regulatory filing in the US which won’t feature the list.
David Cameron and George Osborne have attended fundraising events in recent weeks, held by the BSiE.
Vote Leave have secured significant handouts from prominent hedge fund manager Paul Marshall and CMC Markets boss, Peter Cruddas.
British and Australian businessman Michael Hintze and London based hedge fund manager Crispin Odey have also been linked with possible helping hands to Vote Leave, highlighting the length that backers are dominating funding of rival campaigns.