Liberal Democrat deputy leader Simon Hughes has said his party's backbenchers should have a veto over coalition policies.
Speaking to the BBC, Hughes said Lib Dem MPs should be able to say to the Conservatives "No, we can't go down this road".
"If the coalition wants to deliver [parliamentary] votes, neither party on its own has a majority, so we have to make sure everyone is brought into that,” he said.
"As matter of practical politics... the parliamentary party on behalf of the wider party on big issues has to be able to say, 'No, we can't go down this road'."
Hughes, widely seen to be on the left of his party, also floated the idea that the Lib Dems could still at some point form a coalition with Labour, arguing it was still "on the agenda".
With a large proportion of the Lib Dem parliamentary party now holding ministerial office, Hughes appears increasingly to be adopting the role of the voice of the backbenches.
Since his election as deputy leader in June he has made a number of comments that must have ruffled feathers in Downing Street.
At the start of August he spoke out against David Cameron's suggestion that council tenants may no longer have a "home for life".
Hughes warned that the policy proposed by the prime minister had not been discussed with the Liberal Democrats.
"We have to be clear, it is not a Liberal Democrat policy, it is not a coalition policy, it was not in the election manifesto of either party, it was not in the coalition agreement," he said.
"If he wants to pursue it then there are the proper channels to do so."
He added: "The ideas put forward by David Cameron this week in no way represent the policy of the coalition and certainly do not represent the policy of Liberal Democrats.
And last weekend he ruled out any electoral pact with the Conservatives at the next election, amid suggestions the Tories might not run candidates against vulnerable Lib Dem ministers.
Hughes' latest intervention come as a series of polls appear to suggest the Lib Dems are struggling to retain support despite wider approval of the coalition as a whole.
Analysing an ICM poll conducted for the Guardian to mark the coaltiion's first 100 days in power, the paper suggests that the coalition may be "polluting" the Lib Dem brand while "purifying" the Conservatives.