This is probably the most famous lie, and most leave voters will admit to this.
The EU does not cost the UK £350m a week in any sense. The UK *would* pay £350m a week if its contributions were calculated in the same way as every other member state, but they're not.
If you manage to get a special deal on a new car with an RRP of £15,000, so that you only pay £10,000, you can't go around telling everyone that you spent £15k on your car just because that's the price everybody else pays.
The UK does not pay the "RRP" for the EU. One of the reasons why the UK's current deal with the EU is the best deal of any member states is that it gets a huge reduction on the membership fees that no other members get.
The UK actually pays a gross value of £252M per week. Out of this, £87M comes straight back to the UK to pay for EU projects in the UK.
This leaves a net value of £165M.
It would have been perfectly acceptable to use either the gross or net values, but instead, the Vote Leave campaign used a lie. Vote Leave were instructed by the electoral commission to retract this lie and to stop repeating it. But they didn't.
Unfortunately, the electoral commission has no power to pause, postpone or cancel referenda when one of the official campaigns lies and refuses to stop making a lie that they've been specifically told to stop.
But £252M per week still *sounds* like a lot of money to the average observer, but on the political scale, it's quite small. Approximately 1% of government spending (see the graph, right).
And unfortunately, you can't make the argument that by leaving the EU, we'd be able to spend that money on the NHS, or anything else for that matter. Even ignoring the massive economic hit that will dwarf the EU contributions, the rest of that money (and more) would be needed to duplicate the services currently pooled with the EU.
This raises an obvious question though. If the actual gross and net figures still *sound* high to the average observer, why didn't they just make the exact same (ludicrous) argument with the genuine figures?
The Vote Leave strategy was to discredit the facts from the Stronger In campaign by flooding the debate with lies. If one side is telling lots of lies (and they're allegedly respectable politicians), this doesn't just discredit them, it also makes people assume that the other side are also openly lying. It serves to discredit the opposition, even when they don't tell any lies at all.
By presenting so many lies in quick succession it also kept the media busy trying to fact-check everything. But unfortunately, the way that the media works, the press releases are announced in the morning, when everyone is watching, listening to, or reading the news, and the fact checking takes all morning, so the rebuttals are announced at lunchtime, when very few people are watching or reading the news. By the evening, when people are watching the news on TV, the news channels have stopped reporting it because it has already been debunked.
So people hear the lies, don't hear the rebuttal, and end up believing it.
If Vote Leave had used the correct figures, the media would have been focused on pointing out the underlying flaws in the argument, and that doesn't help the Leave campaign at all.