Almost exactly four years ago to the day David Cameron threw down the gauntlet to European Union leaders in his historic Bloomberg speech.

He announced he would seek a 'new settlement' for Britain in Europe, promising to win a host of concessions from Brussels that would convince Britons to remain in a newly-invigorated Europe.

Their failure to deliver enough concessions backfired, and the European Union has watched Britain walk towards the exit door.

This time the scene of Theresa May's big European Union moment was Lancaster House – a mansion overlooking St James's Park.

It was the place Margaret Thatcher made her iconic 1988 speech extolling the virtues of the single market.

But Theresa May's much anticipated Brexit speech could yet overtake it as the defining moment in the building's history.

Like David Cameron, Theresa May also threw down the gauntlet, announcing Britain would be willing to walk away with no deal.

But this time she is asking for much more than David Cameron.

It came with a stern reminder to those listening - including many diplomats who will be part of the negotiating process - that a lack of flexibility in Brussels could have consequences for them too.

'While I know Britain might at times have been seen as an awkward member state, the European Union has struggled to deal with the diversity of its member countries and their interests.

'It bends towards uniformity, not flexibility. David Cameron's negotiation was a valiant final attempt to make it work for Britain...but the blunt truth, as we know, is that there was not enough flexibility on many important matters for a majority of British voters.'

Her ultimatum has certainly fired the starting gun on negotiations.

It was a speech that needed to take place after months of speculation. Businesses who may have pinned their plans on staying in the single market, will now know for certain that it is an unlikely outcome.

Immigration will be back under Britain's control and that will mean leaving the single market, she said and possibly the customs union too.

The prime minister also confounded critics, including The Economist, whose headline marking her six months in office, characterised her as Theresa Maybe. She was much more forthright than many expected.

The speech was the easy bit.

Delivering free trade arrangements with EU states still smarting is going to be her test.

The European Union called David Cameron's bluff and found Britain was willing to leave, will they call Theresa May's bluff too?

There was a lighter moment in the House of Commons as MPs debated Theresa May's speech.

Anna Soubry - an strong advocate of the single market and free movement of people - very nearly inadvertently referred to the Prime Minister as 'Her Majesty'.

The minister for leaving the European Union David Davis' joke that he often makes the same mistake brought the house down.

The jokes with the most truth are often the funniest.

In Conservative ranks, there is still an air of deference to their leader. And when it came to us, it certainly felt like a royal visit.

In fact, it seemed we were kept even further away from the premier than we would be kept from the monarch.

We were summoned to Lancaster House well over an hour before the prime minister was due to arrive. Far from giving us an hour to enjoy the delights of the historic house and to learn about its wonderful art in many high-ceiling rooms, we were ushered into a basement. Journalists were only allowed out a few minutes before the prime minister was due to speak and then directed back out of the basement door.

Our minders were particularly jumpy about us going anywhere near any diplomats.

Because, god forbid, anyone disagrees with Her Majesty Theresa!