David Cameron and Nick Clegg must draw up rules to stop public splits on government policy after a spate of clashes, peers have warned.
A convention that means all ministers must defend official plans or quit - known as collective responsibility - has been repeatedly breached since the last general election, the House of Lords Constitution Committee said.
Although disagreements are inevitable in a coalition, ministers must reach a collective view "wherever possible" because MPs must be able to hold the Government to account, the Constitutional Implications of Coalition Government report said.
Peers highlighted the decision by the Liberal Democrats to thwart plans for a review of parliamentary constituency boundaries - thought to be worth about 20 extra seats in the Commons to the Tories.
The Deputy Prime Minister initially supported the changes as part of a package of constitutional reforms, but announced in 2012 that his party planned to delay the review after the Tories forced the abandonment of plans to reform the House of Lords.
Peers also pointed to a Tory amendment tabled on the Queen's Speech, which sets out the Government's legislative programme, expressing regret at the decision not to include an EU referendum Bill. The Prime Minister allowed a free vote - a move that some experts warned had constitutional implications.
When both sides cannot agree, a formal process - to be drawn up now by Mr Cameron and Mr Clegg - should be put in place to set aside collective responsibility on the issue, which should be backed by Cabinet and usually include a specific time limit, the committee recommended.
But the move should be rare and only ever used as a "last resort", it warned.
Committee chairman Baroness Jay said: "In any coalition the parties in government will not agree on everything. When those disagreements can be foreseen and included in a coalition agreement this should not threaten the important principle of collective responsibility.
"However, when unforeseen disagreements emerge, such as over a possible EU referendum and the proposals to amend House of Commons constituencies, it is important that an agreed process is in place to ensure disagreements are properly managed.
"We are calling on the Prime Minister and Deputy Prime Minister to agree a set of rules for those situations now. That would give clarity on what will happen for the rest of the parliament and provide a template for future coalition agreements in the event of another hung parliament."
The report also calls for new guidance to be drawn up for civil servants to set out how they should support ministers from different parties, including giving confidential briefings to allow policy to be developed for the next parliament.
Any future coalition negotiations should be "concluded as quickly as possible" and there should be an expectation that the incumbent prime minister remains in post until there is a clear successor, it adds.
Baroness Jay added: "These and a number of other issues should be addressed by political parties and the government before the next election."