Ministers are heading into a stormy period of legislating, in which several key bills risk falling and others are likely to cause splits in the coalition.

Two controversial executive bills reach their first parliamentary vote this afternoon, both facing the risk of rebellions that could derail them.

The Prostitution (Public Places) Bill is intended to outlaw kerb crawling but its failure to make clear if it will do so has raised alarm among MSPs.

Of more political concern is that LibDems are concerned the bill could be ambushed by Labour MSPs at later stages, removing the ability of police to designate management zones for street prostitution, as has happened in Aberdeen and Edinburgh.

Holyrood's legislating process has proved vulnerable to late amendments that can suddenly change key provisions at a late stage and LibDems MSPs last night debated whether they should avoid that risk by voting down the bill today.

They decided against, but the issue is expected to return to cause coalition tensions in coming weeks as the bill goes through its committee stage.

The other bill at risk of defeat last night is the Vulnerable Groups (Scotland) Bill. Both LibDem and Labour back benchers have been threatening revolt, even after ministers had given major concessions.

Rebels in both executive parties want to reduce the burden on voluntary groups to have all their long-serving workers vetted as potential dangers to children and vulnerable adults.

They have also been demanding more detail on how the law will work but Robert Brown, the Children's Minister steering the bill, says that will not be possible by the March deadline when parliament is dissolved.

The LibDem group meeting last night agreed to let the bill go forward but, on that too, future battles between front and back benches are expected.

Other legislation which is causing ministers problems include a back bencher's bill forcing larger shops to give their workers a holiday at Christmas and New Year.

In a bizarre episode yesterday, Johann Lamont tabled amendments but refused to move them, because Labour and Lib-Dem ministers are not agreed on whether to include January 1.

A Labour back bencher on the Justice 2 Committee moved them instead, and all the compromise amendments were defeated, making it more difficult for the coalition partners to find common ground when the bill returns to a full parliamentary vote.

Yet more trouble is brewing with the Labour back-bench bill which would require all health boards to have a majority of directly elected members.

Ministers are opposed but Labour back benchers have allied with other parties to support it and it is unclear if ministers can avoid a defeat when the bill faces a vote at the end of this month.