Chris Mason: Can people believe what No 10 is saying?

This is all about one thing: truth.

Stand back from the swirl of detail and allegations, and it boils down to whether people can believe what No 10 is saying.

And when I say people, of course, firstly that means the electorate.

But it also means the very ministers, members of the government, sent out to explain what the prime minister knew and when.

Grounded into how that works, normally at least, is that they are briefed on the lines to take in response to particular questions.

They have to assume that what they are being told is accurate and truthful.

To put it politely, Downing Street's response to the allegations of recent days has evolved.

Various iterations of it have, it would seem, turned out to be drivel.

And this matters because while this current row relates to the alleged behaviour of the former deputy chief whip, the timeline rings one or two bells for those familiar with all those rows about what became known as 'partygate'.

The questions are all about what Boris Johnson knew, and when.

And the answers continue to change, often in response to awkward facts that prove their previous defence was rubbish, or at the very least not as a candid as it could have been.

The intervention by Lord McDonald this morning was quietly devastating.

Westminster is full of people who love shouting their mouth off.

Lord McDonald is not one of them.

A former senior civil servant not naturally attracted to microphones and cameras.

A man who in his interview on the Today Programme on Radio 4 had the demeanour and hesitancy of someone reticent to be talking publicly.

But a man who felt he had no other options, because, as he put it, No10 "are still not telling the truth".

And then, his most stinging line of all - accusing Downing Street of "telling the truth and crossing your fingers at the same time".

Day by day this means ministers are being sent out with lines to take that disintegrate within hours of them being said out loud - not only humiliating for the ministers concerned but toxic for trust in the leadership of the party, and the country.

And bubbling away underneath, increasing exasperation, from the cabinet down, within the Conservative Party, of the ongoing inability of Number 10 to actually focus on the business of government, rather than constantly firefighting crises about the prime minister's capacity to tell the truth.

Senior ministers who until very recently, the last 10 days, had assumed Boris Johnson would lead them into the next general election now aren't so sure.

And even those still supportive are worn down by what they see as the persistent own goals from Downing Street, which they regard as hard, even impossible to defend.