The Tories are a wealthy political party, with many billionaire donors who want the UK to stay in the EU and there have been worries that without 'purdah' rules in place they could swing the 'stay or leave the EU' vote their way (some would say like buying the 2015 General Election).
Thirty-seven 'Eurosceptic' Tory members of the Commons joined forces with opposition MPs from Labour and the SNP to reject proposed rules for a European Union membership referendum.
The vote was tight at 312-285 but it was Cameron's first defeat since winning the May 2015 election. It should be viewed by Cameron as a lesson from history-Tory party splits over Europe does not bode well for Cameron if you consider Conservative party history.
The Conservatives may have formed a majority government but their grip on power relies on party members obeying the party-line and opposition parties disagreeing among themselves.
But even if a bill scrapes through the Commons it could easily be defeated in the House of Lords; Cameron is trying to redress the Labour Tory balance in the Lords but at this time his party are outnumbered even though the Lords is stuffed to the rafters with members draining taxpayer money.
Mr Cameron's lost vote:
Any guesses how soon before the Tories try to remove 'purdah' rules from law?
Tory pro EU MPs were happy with Cameron's proposals before they were dismissed Monday in the lost vote. For once I will let a Tory have the last word via BBC News as Cameron was accused by "Tory Eurosceptic MP Sir Edward Leigh of offering "legalistic claptrap" in a bid to avoid a Commons defeat. Sir Edward said the process must be "considered to be fair" and argued for Labour's amendment - which reinstated the full purdah rules in the bill - to be accepted."