What Philip Rutnam's departure shows about Johnson’s argument with Whitehall, writes Robert Peston

The fundamental issue revealed by the resignation of the Home Office's Permanent Secretary Sir Philip Rutnam is the yawning gap between what Boris Johnson and Dominic Cummings want post-Brexit UK to be on January 1, 2021, and what senior civil servants think is deliverable.

The PM and his chief aide want to have a fully functioning new immigration system by then, whereas officials fear there’s not enough time.

Johnson and Cummings argue the police should be able to keep us safe if we are no longer part of European Arrest Warrant system. Officials can’t concur.

• Will Boris Johnson and Dominic Cummings be knocked off course by Sir Philip Rutnam's resignation?

• Top Home Office civil servant quits over 'vicious and orchestrated campaign'

• Priti Patel criticises 'false allegations' amid claims of bullying and distrust

Downing Street thinks we can ward off pandemics if we withdraw from the EU’s Early Warning and Response System. The department of health is not so sure.

Even the legendary intellectual dexterity of the mandarin class - Sir Humphrey - struggles to give ministers the advice they crave about the new likely post-Brexit health, scientific, security, law-and-order, immigration settlements (inter alia) - namely that there will be no degradation from the status quo and that the delivery timetable is realistic.

If you are on Cummings side of the road, the problem is Whitehall defeatism.

In Whitehall itself, the crux is that politicians have no practical experience of managing or delivering anything.

This conflict between Downing Street and Whitehall will define Johnson’s first term in office.

PS - a Downing Street official tells me Home Office officials have not told Johnson and Cummings that the immigration reforms cannot be delivered by the due date, and they’ve had the opportunity in two big meetings.

According to my sources, that is bad news for Downing Street delayed.