Eurosceptics have brought down prime ministers, dismantled governments and generally been a thorn in the side of leading politicians for decades.
In finally giving them the referendum they craved, David Cameron has, for now, amplified their voices further.
But who are the Eurosceptics, and why do they want to leave the European Union?
There are two key groups who tend to support a Brexit: older people and poorer people.
The impact of this can be seen in the likes of Clacton – the only parliamentary seat held by UKIP, whose local authority of Tendring is rated as the most Eurosceptic in Britain according to our Sky Data Brexit map.
In this Essex constituency, there are more pensioners than full-time workers, and more than half of voters are on incomes of less than £15,000 per year.
The factors which indicate someone supports a Brexit also manifest themselves in other, less obvious ways.
For example, Sky Data analysis shows Waitrose shoppers are more likely to vote Remain, while those who shop at Aldi are more likely to vote Leave.
Luxury car owners are more likely to be Europhiles, while those who drive small utility cars tend to be more Eurosceptic.
And more obscurely, if you like going fishing, you’re more likely to support the Out campaign – with the reverse being true of cinemagoers.
The main issue driving Leave supporters is immigration.
Two in three Britons (63%) say immigration has had a negative effect on British culture.
The EU is also seen as having a particularly negative impact for unskilled British workers, for whom 42% of Britons think the EU is a bad thing, compared with 16% who think it’s beneficial.
In short, poorer people are worried about their jobs and think their wages are being undercut, while older people feel alienated and intimidated by a multicultural, polylingual Britain that they don’t recognise.
On the other hand, data from YouGov shows the EU is seen as a good thing for our influence on the world stage, employment in the UK generally, and keeping prices down.
For now, neither side is winning the argument on the terrorist threat, nor on the economy – though we shall see if this week’s Treasury forecast of a Brexit costing each UK household £4,300 per year has an impact.
The fact only half of Britons say they have enough information to make an informed decision suggests that opinions may yet be changed ahead of June’s referendum.