The division between Sunnis and Shias is almost as old as Islam itself – and although the two share many beliefs and practices, political and theological divisions have become much more prominent in recent years.
Sectarian violence in Pakistan and the Middle East, and the rise of a brand of Islam that will tolerate only its own interpretation of the faith, have contributed to a polarization between the two communities.
In the first of two programmes, Shaimaa Khalil – herself a Sunni Muslim from Egypt – ventures into the world of Shia Islam and finds out how recent political events have shaped relations between the two communities in Britain.
She joins a Shia mourning ritual, and meets a Sunni-Shia couple who – after marrying for love against their parents’ wishes - are negotiating a difficult path between mutual understanding and the need to live out their own faith.
Shaimaa also consults Sunni and Shia experts - Tim Winter from Cambridge University, and Sheikh Mohammad al-Hilli - to get to the bottom of what has remained the most divisive issue in early Islamic history: the succession of the Prophet Muhammad.
And, she meets a Shia bridge builder from Pakistan, Rubab Mehdi Rizvi, who at the age of 13, deflected a bomb threat from Sunni extremists by persuading the wife of the Saudi ambassador to act as a human shield.