In a vast college sports hall in southwest London, Theresa May's worst week as Prime Minister has gone from bad to truly terrible.

Even though he wasn't officially a Conservative candidate, Zac Goldsmith's defeat by the Liberal Democrats is a disaster for the Tories.

The defeat - in a by-election the Lib Dems turned into a referendum on Brexit - capped a horrible week for Mrs May, with a Brexit embarrassment every day:

:: On Monday, notes carried by an aide to Tory MP Mark Field were photographed and revealed a "have cake and eat it" strategy

:: On Tuesday, a brusque letter from Donald Tusk to MPs on ex-pats and EU citizens in the UK said the MPs were not living in "reality"


:: On Wednesday, Boris Johnson's unguarded remarks to ambassadors on the single market spread confusion about Government policy

:: On Thursday, David Davis suggested the UK might pay Brussels a fee for staying in the single market, prompting fury from Eurosceptics

:: Then on Friday, shortly after 2am, Sarah Olney won a spectacular by-election victory and claimed - with good reason - it was Brexit wot won it.

Olney victory a rejection of 'anger and division'

In her victory speech, she said she had never been involved in politics until after the Lib Dems were all but wiped out by the Tories in the 2015 general election.

She beat a high-profile Tory with massive name recognition in his own constituency who won by 23,000 in 2015 and was the Conservative candidate for London mayor.

Short of being a senior Cabinet minister or Opposition spokesman, you can't get more high profile than being one of the big parties' candidate for mayor of the capital.

Was that a disadvantage for Mr Goldsmith, though?

Labour's candidate Christian Wolmar, in a rather ungracious speech from the platform, suggested that his mayoral campaign - in which Mr Goldsmith was accused of racism against Labour's Sadiq Khan - damaged him in the by-election.

After the result, Mr Goldsmith was distraught.

His voice shaking, he told me he didn't regret resigning and triggering the by-election. I'm sure he means it, but he is devastated.

As he left the count, he told me his fight against Heathrow expansion will go on. But he won't be able to do that in Parliament now.

When I asked him if he thought he could bounce back from this defeat, he was silent, as his wife Alice put a consoling arm around his waist.

One of his supporters told me he would bounce back. But I wonder. Have bruising defeats in the mayoral election and now in this by-election left him disillusioned with politics and ready to call it a day?

For the Liberal Democrats, this result marks a remarkable comeback from their crushing defeats by the Tories in 2015.

They now have nine MPs in the Commons - still not many, but the result will be a huge morale boost for them ahead of local elections next spring and as they begin the long fightback leading to the 2020 general election.

The result will also cheer Remain supporters in other parties and throws Theresa May's strategy into fresh turmoil. The confusion of this week could now turn into acrimony among Tory MPs.

Full declaration of Richmond Park results

Lord True, a former No. 10 policy chief under John Major and now leader of Richmond Council, said he feared un-elected Lib Dem peers in the House of Lords would now feel emboldened to try to block Brexit.

And what about the prospects for a general election next year?

It looked unlikely at the start of this week. Now it looks even less likely, unless - perhaps - the Government loses in the Supreme Court next week, sparking a constitutional crisis.

Tory MPs who won their seats from Lib Dems in 2015 will suddenly be worried. And the Lib Dems will now hope to win many of those seats back.

The Witney by-election six weeks ago, when the swing to the Lib Dems was 19%, was the turning point in the Lib Dem comeback. It encouraged them to believe they could win in Richmond Park.

But they did better than in Witney, with a swing of 21% and a sensational victory that is right up there with some of the Liberals' and Lib Dems' most famous triumphs.

It's only a by-election, the Tory high command will say, and Zac Goldsmith was an independent. But by-elections can sometimes spook governments into a change of course.

Mrs May will be determined to make sure that doesn't happen. Her opponents will fight hard to try and make it happen.

But the Prime Minister won't want many more weeks like this one.