Your mother rang,’ my husband said when I walked in the door. ‘She said, “Is it true that Rachel’s joined the Lib Dems?” It’s the first time I’ve ever heard her sounding angry,’ he went on. ‘You’d better call her.’

My heart sank. My mother and my father are about the two people left in my life in whose eyes I can do no wrong.

She was clearly astonished that I’d elected not to support the super-party of government where two of her sons serve as Ministers, but a rival pro-EU tiddler, thus halving my fanbase of two and also managing to drag my deep, doctrinal differences with Boris into the national spotlight once again, just as a historic General Election – based on securing a national mandate for his own beautiful bouncing baby, Brexit – was getting under way to boot.

Brexit clash: Rachel Johnson at a bash with her brothers Boris and Jo

I knew I had some explaining to do, so I picked up the phone.

‘Hello Mum,’ I said. ‘Is it true?’ she asked, with an edge to her voice. My mother voted Leave out of loyalty to her first-born son and also because she thinks ‘emotionally’ Britain has never been a part of the Continent and we are a precious stone set in a silver sea and all that. ‘And did Al know?’

It was the killer question. For as I was inputting my debit details (£70) for my annual sub, I was committing to the party that – if in power – would undo the passion project of my older sibling Boris (known as Al by family) and kick both my MP brothers out of a job.

I was coming out as fratricidal Lib Sab. Although, I promise, it didn’t seem like that at the time: as I clicked ‘Submit’ it was in the spirit of individual sovereignty, that I was entitled to have a different and strong opinion on an important subject, and even express it.

Yes, I soothed, I sent a text earlier in the week to warn my brother that I’d joined what he calls ‘the coalition of chaos’, making a limp joke about it being time for a futile gesture. He made it quite clear he thought I’d lost my last remaining marble.

I sometimes forget that even though I am not important, anything I do can be made into a story if it can lead to an unfortunate headline for my more famous brother, especially if my ‘bombshell’ breaks as he’s touring the TV studios, promoting his party’s strong and stable leadership message and explaining what ‘mutton-headed mugwump’ means.

Yes, with hindsight, my apparent defection from the political family fold (one US website called it ‘an extraordinary rebuff’ and I winced) was irresistible copy: the Milibands in blond wigs, all over again.

Some newspapers have called me a turncoat, fickle, disloyal, even a mugwump, whatever that is

And I accept full responsibility for this. I could have told nobody I’d joined the Lib Dems. I could have merely shown my allegiance in the privacy of the voting booth, but I didn’t.

I wanted to stand up and be counted in my continuing opposition to what I’ve always thought is a suicide mission to take us out of Europe and over a cliff.

Many kind souls have suggested I should go even further and fight a seat in this Election, but that can’t and won’t happen – there are rules, there are procedures, it’s all very… Lib Dem, shall we say.

But I’m glad I have the platform to say this, here, as I can’t be a prospective candidate, to explain. And it’s very simple.

Some newspapers have called me a turncoat, fickle, disloyal, even a mugwump, whatever that is.

I’ve heard from Tory high command that me going ‘libtard’ went down like a bucket of cold sick on the stump, in their constituencies challenged by the yellow peril especially.

But I joined the Lib Dems (‘for now’, as George Osborne said of leaving his seat of Tatton) because they are the only party that represents me, and the 48 per cent, 16 million others, when it comes to this Election and the biggest issue of my lifetime.

I continue to think that the referendum result turned a peachy win-win position vis-à-vis our 27 former EU partners into a lose-lose overnight.

And meanwhile nobody – and I mean nobody – has ever persuaded me of a single advantage of leaving the EU. Not one, apart from what I call ‘3B’ type benefits (Blue passports, the yacht Britannia, and Bluebirds Over The White Cliffs Of Dover, that sort of thing).

As I say to my brother every time I see him – and he would, I hope, admit I’ve been consistent if not loyal on this – the reason I’ve gone as far as to join an opposition party is because the goods the Brexiteers showed us in the referendum bill of sale have so far turned out to be faulty.

He said we’d be in the front of the queue when it comes to a trade deal with the US and we’re behind the EU. There is to be no regular injection of £350 million for the NHS.

We are leaving the single market to take control of our borders, but one of the few things David Davis has said, as Brexit Secretary, is that we need migration to thrive and most company bosses agree (Barclays last week said that immigration was a far bigger deal for the City than financial passporting).

Growth is slowing, with GDP predicted by the CBI to be down five per cent by 2020, and on it goes – and Brexit hasn’t even happened yet.

We are leaving the single market to take control of our borders, but one of the few things David Davis has said, as Brexit Secretary, is that we need migration to thrive

And therefore for me, this time round, the only way is the Lib Dems, and for this reason: when it comes to Europe, Boris is wrong on this, and they are not ‘wobbling jellies of indecision’.

In fact, they have the only sensible plan in place. They are campaigning against a ‘hard Brexit’ that would take the UK out of the single market and end free movement of people – and for a second referendum on the terms of any Brexit deal reached with the EU.

With that offer on the table, I can’t in all conscience support a party (and I write as a former economics hack on the FT and past Tory voter) that’s committed to sacrificing prosperity on the altar of a false and unfeasible pledge to bring immigration down to the tens of thousands, a target that the Office for Budget Responsibility says will cost £6 billion a year.

‘Are you angry with me too, Mum?’ I said, as I finished explaining all this in pleading tones.

‘No, I’m not,’ she sighed. ‘You seem to forget I was a founder member of the SDP.’

In the middle of all this, my husband returned from abroad. I should say with him I’m usually in the doghouse too.

He doesn’t do Twitter, but on this occasion he tweeted.

It took several attempts, but he said in 140 characters what I’ve just tried to say in 1,400 words.

‘It’s been 25 years since my wife and I agreed on anything – but then Brexit. I joined the Lib Dems six weeks ago,’ he tweeted. ‘Why haven’t you?’