Image source, Jabin Botsford/The Washington Post via Getty Images

Sondland will say he, Energy Secretary Rick Perry and former Ukraine envoy Kurt Volker reluctantly worked with Trump's personal lawyer, Rudy Giuliani, "on Ukraine matters at the express direction of the President of the United States".

He will testify that Giuliani wanted Ukraine to launch an anti-corruption investigation into a Ukrainian gas firm, Burisma, where Democratic White House contender Joe Biden's son worked while his father was US vice-president.

Sondland will say: "Mr Giuliani’s requests were a quid pro quo for arranging a White House visit for President Zelensky.

"Mr Giuliani demanded that Ukraine make a public statement announcing investigations of the 2016 election/DNC server and Burisma.

"Mr Giuliani was expressing the desires of the President of the United States, and we knew that these investigations were important to the President."

The BBC's Anthony Zurcher sums up the blockbuster testimony from EU Ambassador Gordon Sondland: everything Sondland did, "the Ukrainian pressure, the quid pro-quo - was at the direction of Donald Trump".

Or, as BBC North America Bureau Chief Paul Danahar says, Sondland has thrown the President, Secretary of State Mike Pompeo and Rudy Giuliani "under the bus hurtling towards him".

Image source, Getty Images

At the heart of the impeachment hearings has been a latin phrase "quid pro quo", meaning an exchange of favours.

It is the alleged smoking gun that congressional Democrats have been trying to adduce during their investigation of President Trump.

Sondland has a lot to say about this in his opening statement, which we have a copy of.

According to his prepared remarks, he will say: "I know that members of this Committee have frequently framed these complicated issues in the form of a simple question: Was there a 'quid pro quo?'

"As I testified previously, with regard to the requested White House call and White House meeting, the answer is yes."

The wealthy hotelier appointed as ambassador to the EU by Trump has found himself at the centre of the impeachment inquiry.

Diplomats have described Sondland, along with other officials, as being part of a shadow foreign policy operation on Ukraine, sidestepping official channels to pressure Zelensky to investigate Biden and his son, Hunter Biden.

In closed-door testimony last month, Sondland said he did not know about any preconditions being attached to military aid for Ukraine.

But he revised that testimony on 4 November in a three-page addendum to reflect memories he said had been refreshed by other witnesses.

In particular, he said he remembered a conversation with Andriy Yermak, an aide to the Ukrainian president.

"I now recall speaking individually with Mr Yermak, where I said resumption of US aid would likely not occur until Ukraine provided the public anti-corruption statement that we had been discussing for many weeks," Sondland said.

There are quite a few faces in the hearing room. Here's who you need to know.

First, the lawmakers...

And their lawyers...

And, the key witness:

It's a complicated story. We’ve made it a little easier to digest.

SIMPLE GUIDE: If you want a basic take on what's going on, this one's for you

GO DEEPER: Here's a 100, 300 and 800-word summary of the story

WHAT'S IMPEACHMENT? It's a political process to remove a president - video guide

VIEW FROM TRUMP COUNTRY: Hear from residents in Pennsylvania

ON THE DOORSTEP: A newly elected Democrat sells impeachment to voters

CONTEXT: Why Ukraine matters to the US

Welcome to our live coverage of the impeachment investigation into President Donald Trump.

Today, lawmakers will hear from US Ambassador to the EU Gordon Sondland - a key witness who could shed light on how connected the president is to the Ukraine matter.

Sondland has already testified once behind closed doors, and then revised that testimony after his memory was refreshed by other witnesses, he said.

And the question at the heart of the inquiry - whether President Donald Trump abused his power by pressuring Ukraine to investigate a political rival.

We’ll keep you updated on all the latest happenings on Capitol Hill here.

Image source, Getty Images