Justice Secretary Michael Gove has claimed the Prime Minister's EU reforms are not legally binding and could be overturned by European judges.
David Cameron has said the agreement he secured in Brussels is "legally binding and irreversible"
But in an interview with the BBC, Mr Gove said the European Court of Justice was "not bound" by the agreement agreed by Mr Cameron until treaties are changed.
"The facts are that the European Court of Justice is not bound by this agreement until treaties are changed and we don't know when that will be," he said.
David Cameron has warned that Britain has "100 days to secure our future", as a poll suggests the final result hangs in the balance.
The EU referendum is a choice between "the devil and the deep blue sea", one of the UK's most senior business leaders is to say today.
Britain's car industry is an economic success story, and with uncertainty over the impact of Brexit, it wants to stay put.