Context : The dominating reionization source in the young universe has not yet been identified .
Possible candidates include metal poor dwarf galaxies with starburst properties .

Aims : We selected an extreme starburst dwarf , the Blue Compact Galaxy Haro 11 , with the aim of determining the Lyman continuum escape fraction from UV spectroscopy .

Methods : Spectra of Haro 11 were obtained with the Far Ultraviolet Spectroscopic Explorer ( FUSE ) .
A weak signal shortwards of the Lyman break is identified as Lyman continuum ( LyC ) emission escaping from the ongoing starburst .
From profile fitting to weak metal lines we derive column densities of the low ionization species .
Adopting a metallicity typical of the H II regions of Haro 11 , these data correspond to a hydrogen column density of \sim 10 ^ { 19 } cm ^ { -2 } .
This relatively high value indicates that most of the LyC photons escape through transparent holes in the interstellar medium .
We then use spectral evolutionary models to constrain the escape fraction of the produced LyC photons .

Results : Assuming a normal Salpeter initial mass function we obtain a Lyman continuum escape fraction of f _ { esc } \sim 4–10 % .
We argue that in a hierarchical galaxy formation scenario , the upper limit we derive for the escape rate allows for a substantial contribution to cosmic reionization by starburst dwarf galaxies at high redshifts .

Conclusions :