David Sullivan and David Gold have both increased their shareholdings in West Ham United from 25 per cent to 30 per cent, taking the proportion of the club they now jointly own to 60 per cent.
Mr Sullivan and Mr Gold, who each bought 25 per cent of the club in January, purchased 10 per cent from CB Holdings - the previous Icelandic owners - for £8m, with £4m going to the club and £4m going to CB Holdings.
Mr Sullivan said the investment illustrated the Chairmen's commitment to the club's long-term future.
"Myself and David Gold are, like you, West Ham United fans and our aim is to ensure our club enjoys a stable and successful future. To this end, we have both invested a further £4m to increase our shareholding to 30 per cent each.
"We have a long-term intention of increasing our shareholding, while we have also invited professional investors to share in our vision for this special club. Also, in the near future we hope to extend to supporters the opportunity to invest in the club.
"We are committed to putting West Ham on a firm financial footing that will form the basis, we hope, for success both on and off the pitch."
CB Holdings still own 40 per cent of the club, although the Chairmen have a three-year option to buy these shares.
The £4m that goes to West Ham will bridge the gap in the club's finances.