Artifacts at skybox cube corners

I have a cubic skybox: all the six textures are perfect, but i get artifacts at corners, you can see that quads seems separated even if they are perfectly matching one to each other.
Here is a screenshot: http://www.web-discovery.net/artifacts.jpg

Do i have to switch to a sphere skybox or there is a solution to this ?

GL_CLAMP_TO_EDGE

i do use it as it follows:

glTexEnvi(GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_MODULATE);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE);

(just quickfix idea, so discard if you want perfect solution)

it probably originates in low precision of
some depth data or things like that,
so when you adjust the sizes of sides of the cube that they will overlap each other just a little, you may get the desired result