Boris Johnson grabbed most of the newspaper front pages this morning after announcing he will be campaigning for Britain to leave the EU.

The outgoing Mayor of London wrote a long article in the Telegraph explaining why he thinks Britain would be better off out of the union.

Did he pass the FactCheck test?

“According to the House of Commons library, anything between 15 and 50 per cent of UK legislation now comes from the EU.”

True. The House of Commons researchers published this authoritative study of how much UK law is made in the EU in 2010, and updated it here.

The percentage could be as low as 15 and as high as 50 because there is a big argument you need to have about whether to count all regulations (which can cover very trivial rules) as “laws”.

Boris has avoided using the very high percentages alleged by other Eurosceptics over the years, notably Ukip.

The party claimed 75 per cent of British laws were made in Brussels, but this was based on a mistaken interpretation of the words of a former European Parliament president. Ukip stuck with the bogus figure even when the mistake was pointed out to them.

“Sometimes these EU rules sound simply ludicrous, like the rule that you can’t recycle a teabag, or that children under eight cannot blow up balloons, or the limits on the power of vacuum cleaners…”

If they sound ludicrous, they usually turn out to be untrue, as Mr Johnson – the Telegraph’s former Brussels correspondent – knows better than most.

He alludes to his time in Brussels in today’s article, reminiscing about covering “euro-condoms and the great war against the British prawn cocktail flavour crisp”.

Both of these claims – that eurocrats were designing a standard-sized condom (too small, naturally, for the average British male) and wanted to ban prawn cocktail crisps – were enthusiastically reported in British newspapers despite being substantially untrue.

The “rule that you can’t recycle a teabag” appears to come from Cardiff council officials blaming Brussels for changes they brought in to try to prevent foot-and-mouth disease. The European Commission said it was up to member states to decide what to do with their tea bags.

Similarly, there is no ban on under-eights blowing up balloons, just a decades-old rule that they should come with a safety warning. The Commission says this is a well-intentioned attempt to stop small children choking to death.

At least there is some truth in the vacuum story. EU energy saving regulations have restricted the “input power” of vacuum cleaners (not the same as the sucking power).

Interestingly, celebrated hoover-improver Sir James Dyson wanted to pull out of the EU following this decision – because it didn’t go far enough in making life difficult for his less energy-efficient competitors.

“…Sometimes they can be truly infuriating – like the time I discovered, in 2013, that there was nothing we could do to bring in better-designed cab windows for trucks, to stop cyclists being crushed. It had to be done at a European level, and the French were opposed.”

This sounds like Boris, as London’s cycling-friendly mayor, wanted to redesign lorries to make them safer, only to be thwarted by the EU, and the French in particular. But this story doesn’t fit the facts.

In 2014, the European Parliament voted overwhelmingly to change the shape of lorry cabs to cut cyclist deaths, despite initial opposition from some national governments, including that of the UK.

We know this because one B Johnson was a big cheerleader for the EU-wide changes, and a critic of the Conservative-led government’s stance at the time.

He was quoted as saying: “If these amendments, supported by dozens of cities across Europe, can succeed, we can save literally hundreds of lives across the EU in years to come. I am deeply concerned at the position of the British government and urge them to embrace this vital issue.”

It’s true that the French and Swedish governments tried to delay implementing the changes for ten years, but they failed, and new regulations will come into force in 2019.

So it’s not true that “there was nothing we could do”. The European Parliament actually implemented the changes backed at the time by Boris himself. It’s hard to see why he’s criticising the EU over this now.

“Most of the evidence I have seen suggests that the Scots will vote on roughly the same lines as the English.”

We don’t know what evidence Boris has seen on this, but it goes against all the polling analysis we have seen.

A recent study by NatCen Social Research found Scots were significantly more likely to vote to stay in the EU than the English – 64 per cent compared to 52 per cent.

And this wasn’t a blip – Scotland’s pro-EU bias was visible over at least the last 15 years:

“Only 4 per cent of people running the Commission are UK nationals, when Britain contains 12 per cent of the EU population.”

True. The latest figures are here. Some 4.3 per cent of European Commission staff – 1,000 people – are UK nationals. More than 8 per cent are Germans, nearly 10 per cent are French, and more than 17 per cent are Belgian.

The number of British staff and the proportion have been falling for at least a decade, according to Foreign Office figures:

Almost everyone is concerned about this, but no one blames the European Commission for trying to freeze Brits out. The main problem is that young people are not applying for roles fast enough to replace British staff who retire.

This report from the Commons Foreign Affairs Committee notes that all candidates have to sit language exams, and young Britons are at a disadvantage: “In 2010 6 per cent of upper secondary students in the UK were learning two or more languages, compared to an EU average of 60 per cent.”

What else might be putting British candidates off, the committee wondered? One theory was that uncertainty over Britain’s continued membership of the EU was making the prospect of a career in the EU institutions less attractive.