What an extraordinary week to become the new leader of the Liberal Democrats. Having finally entered No 10, Boris Johnson appointed the most hard-line Brexit government, while Jeremy Corbyn utterly failed to challenge it.

Boris Johnson is not fit to be a prime minister. A prime minister wouldn’t compare Muslim women to letterboxes, or describe elite women athletes as “glistening like wet otters”, and he or she wouldn’t continue to threaten a no-deal Brexit knowing full well the damage it will do. Whether it is throwing people under the bus, as he did with Sir Kim Darroch, or writing a lie on the side of one, one thing is clear: Britain deserves better than Boris Johnson.

I can’t say I was surprised when I saw who the top jobs in his government went to. We have the full range of extreme, illiberal views represented in the cabinet – among them we have someone who has supported the death penalty, another who thinks abortions should be banned even in cases of rape, and another who wants to scrap the Human Rights Act. This is not the modern Britain cabinet that we were promised. Britain deserves better than this.

Jeremy Corbyn had yet another opportunity to prove that he can be the leader of the opposition. And he bottled it.

Faced with the hardline Brexit government that Boris Johnson appointed, Corbyn did nothing. On Wednesday I wrote asking him to table a motion of no confidence, but he didn’t. Yet another test of Corbyn’s leadership, yet another failure. Britain deserves better than an opposition that just doesn’t show up.

In light of all this, I can understand how frustrated people across our country will be feeling. Britain deserves a better choice than Johnson or Corbyn.

We deserve the opportunity to vote for a positive, alternative vision for our country. One where we work with our closest allies to tackle the big challenges we face, where our economy works for people and our planet, and one where – no matter who you are, whatever your background – you can access the opportunities you deserve. That’s what the Liberal Democrats have to offer.

If there are MPs who believe Brexit can be stopped, that Johnson, Corbyn and Farage must be stopped, then work with us

The forces of nationalism and populism have risen in our country with chilling consequences. To beat them we need to rally a liberal movement – bigger and stronger than ever before. I believe that the Liberal Democrats have proved that we are the beating heart of that movement, with our successes at the local and European elections. As the new leader, it is my biggest priority to grow this movement and take on Johnson, Corbyn, Nigel Farage and the rest.

To build that liberal force our country needs, I will work with whoever shares our values in whatever shape or form. The risk to the future of our country is so severe that this is not the time for political tribalism and pettiness. Let me be blunt – if there are MPs who believe Brexit can be stopped, that Johnson, Corbyn and Farage must be stopped, that Britain deserves better, then work with us, join us. My door is always open.

And my message to everyone who sits on the sofa and shouts at the television when watching Johnson’s blustering bravado is clear: politics is not a spectator sport. If you want to stop Brexit, if you want to keep our family of nations united, if you think we can build a richer, greener and safer future, then join me, join us. Together we can do this. Together we can change politics and transform our country.

• Jo Swinson is the leader of the Liberal Democrats party