Talks between German publisher Axel Springer and US private equity investors in ProSiebenSat1 reach a key phase this week as Springer is poised to start due diligence in preparation for taking an increased stake in the commercial television broadcaster.

The two sides have been sounding each other out for months on what would be one of the biggest media deals in Germany in years, and one that would transform Springer, best known for its mass tabloid Bild, into a more widely-based media group.