Israel's ruling coalition is facing an internal power struggle as it attempts to replace a law which allows tens of thousands of ultra-orthodox men to avoid military service – a law that has now been declared unconstitutional by the country's Supreme Court.

Prime Minister Benjamin Netanyahu faces formidable pressures from the secular and religious poles of the coalition as he tries to implement the Court's view that the ultra-orthodox should share more of the burdens imposed on the public at large.

This week's landmark ruling by the Court enjoins Israel's parliament to substitute a "proportionate, egalitarian and constitutional" measure when the decade-old "Tal law" expires in five months time.

The huge growth since the formation of the state of Israel in 1948 in exemptions for young religious men from compulsory service in the Israel Defence Forces has been a factor in a deepening divide between secular and ultra-orthodox, or Haredi, citizens. There are currently more than 60,000 exemptions a year.

Some secular politicians, including the Defence Minister Ehud Barak, have spoken of confining exemptions to only a few thousand ultra-orthodox men, and a major overhaul will certainly be backed by the Foreign Minister, Avigdor Lieberman, who leads the right wing and fiercely secular Yisrael Beiteinu party.

But that might require Mr Netanyahu to forsake his ruling Likud party's links with the two main ultra-orthodox parties Shas and United Torah Judaism.

In a separate conflict, the Chief Rabbi of Tel Aviv yesterday expressed "deep disappointment and pain" at a vote by the municipality this week in favour of buses being allowed to run on the Sabbath.

Supporters of the proposal, including the city's mayor, Ron Huldai, argue that the 24 hour absence of public transport impacts especially severely on poor families without cars who might want to visit relatives or the beach on their only day of leisure. Mr Huldai wrote bluntly on his Facebook page: "Anyone who doesn't want to get on a bus, shouldn't get on."

But the Rabbi, Yisrael Meir Lau, called on the government to reject the municipality vote, declaring: "This is a severe blow to the holiness of the Shabbat, which is a remnant of Creation, a reminder of the Exodus from Egypt, a day of rest for every worker and a day of spiritual ascension and the unity of the family."