In this week in 1910, the British electorate went to the polls. They did so because Herbert Asquith’s Liberal government had been unable to get Lloyd George’s famous People’s Budget through the House of Lords. Liberal posters defined the election as a choice between the peers and the people. They finally got their way after a second election that December.

The fixed-term parliament act was never intended to prop up a zombie government

So twice that year, and a number of other times, governments who could not get their flagship legislation through parliament, or who otherwise found their authority in the House of Commons exhausted, have been obliged to go to the country to seek a new mandate.

For Earl Grey in 1832, it was electoral reform. For Gladstone in 1886, it was Irish Home Rule. And for Clement Attlee in 1951 and Ted Heath in 1974, it was a last throw of the dice when facing parliamentary stagnation and national crisis.

That is how our country works when we have a government that cannot govern; that is the way it has always worked going back to the Duke of Portland in 1807; and that is the situation Theresa May will find herself in after she loses the vote on her Brexit deal.

The reason all those previous examples matter is that – in a country with an unwritten constitution like ours – we are reliant on historical tradition, rather than a clear set of rules, to dictate what should happen in circumstances like these.

And based on that historical tradition, there is no question that the prime minister should respond to tomorrow’s vote – and parliament’s rejection of her proposals on the most vital issue facing our country – by calling a general election. Indeed, it would smack of hypocrisy if a Tory government that complained loudly about the Speaker setting aside parliamentary precedent last week was simultaneously happy to ignore every precedent followed by their own predecessors for two centuries. The Tories must know the government cannot keep trying to limp on, unable to govern and pretending that the will of parliament is irrelevant.

They may claim that the Fixed Terms Parliament Act has changed the unwritten constitutional requirements expected of sitting governments. But again, they know themselves that act was only passed in 2010 to give some stability to the Tory-Lib Dem government, and to ensure that Nick Clegg couldn’t hold David Cameron to ransom on policy issues by constantly threatening to collapse their coalition. It was never intended instead to prop up a zombie government that couldn’t pass its most important legislation, and for any Tory to hide behind that act now in refusing to call an election would be utterly shameful.

Theresa May said last week that our country would be in “unchartered territory” when MPs vote down her deal tomorrow. She is wrong. This is territory numerous previous prime ministers have walked through before.

She has a constitutional obligation to follow the only course of action titans like Gladstone and Attlee felt was practically and honourably open to them when their parliamentary authority fell away.

The prime minister must call an election. Anything else would not be stoicism or stubbornness, or whatever other qualities her admirers claim; it would just be sheer cowardice.

But if she refuses, if Labour’s no confidence motion fails, and if we have to move to other options, including campaigning for a public vote, we will take no lectures from her about respecting our country’s democracy. Because she will be the one who has forced us into that position by ignoring every historical precedent on which that democracy is based.

• Emily Thornberry is shadow secretary of state for foreign and Commonwealth affairs and Labour MP for Islington South and Finsbury