Isabel Oakeshott is writing a biography of David Cameron with Lord Ashcroft and is a former Political Editor of the Sunday Times

This Christmas, one MP deserves a present Father Christmas cannot deliver. Emily Thornberry, the highly qualified former shadow attorney general, should be given her job back, if not in her stocking, at least in the New Year.

Dredging up an old story rather than revealing something new breaks a basic journalistic rule. Columns are supposed to look forward, rather than rake over old coals. However, as the general election approaches, I believe that the row over Thornberry’s tweet matters more than ever. It has major implications for MPs and political campaigners from all parties, and everybody who cares about freedom of speech.

When the flak was flying over Thornberry’s tweet from Rochester last month, I decided to sit it out. Twitter is such an aggressive forum that occasionally I can’t face spoiling my day by taking an unpopular stance. Over the years I’ve grown a thicker skin against the bile of the so-called “haters” on social media, but the viciousness with which some users express disagreement or disapproval still has the power to knock me sideways.

Yet I believe there is an important principle at stake in the Thornberry case, and I am sorry I did not stand up for it – or her – at the time. So, on the basis that it’s better late than never, I want to say, loud and clear, that she should never have quit.

The tweet that did for the former shadow attorney general was posted from Rochester one Thursday afternoon last month in the midst of the by-election triggered by Tory MP Mark Reckless’ defection to UKIP. It showed a decently kept terraced two storey house with a white Transit van on the drive and three St George’s flags festooned over the windows. Thornberry posted the picture with just three words of comment: “Image from Rochester.” Readers could make of it what they wished.

Yet the tweet triggered a furore, with Thornberry accused of snobbery and of insulting and dismissing working class people. Labour leader Ed Miliband was said to be “furious.”

It is hard to know where to begin with this nonsense, but let’s start by pointing out that the only comment Thornberry attached to the image was factual. The picture was indeed an “image from Rochester.” The picture was open to interpretation. Her trial in the court of public opinion was therefore not about anything she said but what she might have been suggesting.

Under heavy fire from an army of bien pensants and political opportunists, Thornberry claimed that she posted the picture because she thought all those flags, which entirely obscured one of the upstairs windows of the property, were a remarkable sight. This is entirely plausible. Whether it is true, only she knows. We cannot tell what was going on in her mind; but in my view, we should not be in the business of speculating. If we start sacking people on suspicion of thinking bad things, where will it all end? It cannot be right for people to lose their jobs over what they might or might not be thinking.

Possibly, Thornberry’s inference was derogatory; but it is equally the case that those who saw the image and assumed it was insulting were applying their own negative value judgments to the picture. If the Thornberry resignation rule applies to everybody, those who criticized the picture could themselves be ejected from their jobs, for what they might have been thinking. This is clearly absurd.

As a barrister, the irony was surely not lost on Thornberry that that none of the accusations leveled at her would have stood up in a court of law. Her crime – if indeed she was guilty of any – was a thought crime. Her punishment is insidious. Do we want laws governing what we think? If so, will suspects undergo brain scans to ascertain whether thought crimes have been committed?

In the run up to the general election, it is more important than ever that politicians should be able to say what they think. Otherwise, how are voters to know who they are electing?

Last week, a Westminster think tank revealed that a third of people in Britain believe they cannot speak freely on controversial subjects such as immigration and religion for fear of being criticised, losing their job, or being prosecuted.

Is this the kind of country we want?