We boobed,” says the latest Marks & Spencer advertising campaign. The high street store has just agreed to stop charging £2 extra for larger bras, after a campaign by a group called Busts 4 Justice. As the chairman, Sir Stuart Rose, admitted: “We were wrong.”

I am not sure whether any MPs have claimed ladies' lingerie on their parliamentary expenses, or indeed whether it was size DD or above, but politicians have certainly been extraordinarily slow to understand the scale of public anger and to apologise for what has gone on.

The row over M&S underwear was, according to the headline writers, a storm in a D-cup. Not so the revelations about MPs' expenses, which are now tapping into a wider public mood of disenchantment with mainstream politics that could threaten our democracy.

Yesterday, contrition seemed to be contagious. No sooner had David Cameron used the “s” word than Gordon Brown rushed to say sorry for the first time since details of the parliamentary practice of “flipping” and claims for flat-screen TVs, fuschias and Farley's Rusks began to emerge. “We have to acknowledge just how bad this is,” the Tory leader said.

The Prime Minister added: “I want to apologise on behalf of politicians, on behalf of all parties for what has happened.” Michael Martin, the Speaker, promised to set up an “operational assurance unit” to monitor MPs' future claims.

But it was too little too late. For days now, Mr Brown has blamed “the system” rather than admitting that mistakes had been made. All weekend ministers insisted that they were obeying “the rules”.

Lord Mandelson said that he was the victim of a “smear” campaign. Labour MPs were e-mailed by party officials telling them that they had done nothing wrong. Mr Brown's apology looked like a tactical attempt to catch up with the Tory leader rather than a heartfelt statement of regret. And even if party leaders are now willing to admit collective guilt it is still far from clear if the political class will follow M&S and reduce how much they charge.

As someone who speaks to MPs a lot, I do not think that politicians are all corrupt or greedy. Most are decent, hard-working people who retain at least some of their idealism. They do not go into politics to make money, they are underpaid compared with other professionals and even the now notorious second home allowance was set up for a reason. It is expensive to live in two places. We cannot, on the one hand, demand that Parliament be more representative and then make it impossible for anyone without a private income to become an MP. Surely it was also wrong for a newspaper to pay a six-figure sum for information that had been illicitly obtained (some would say stolen) from the Commons fees office.

And yet, and yet. I cannot see how anybody could think it appropriate to charge the taxpayer for eyeliner, Tampax, hanging baskets or a baby's buggy. Nor can I understand how a politician could think it proper to have one main residence for capital gains tax and another for his or her exes. Set against such details, even legitimate claims look questionable. MPs need to face up to how the voters feel: politics is as much about emotional intelligence as reason.

Karl Marx said that “history repeats itself first as tragedy second as farce”. In the 1990s, politics was damaged by the news that MPs were taking cash in brown envelopes to ask questions, now it is being undermined by revelations that politicians ask the taxpayer to cough up for glitter loo seats, Jaffa Cakes, horse manure and mock Tudor beams. The whole thing would be ludicrous if it did not also reveal a sense of entitlement pervading the Palace of Westminster that creates a dangerous gulf between the electors and the elected.

A couple of years ago a minister told me that he had had a row with the Commons authorities after trying to submit a full set of receipts for his expenses (at the time only claims above £250 had to be accompanied by a receipt). He was told he would be setting a dangerous precedent for his colleagues. Parliament, led by the Speaker, has been woefully reluctant to modernise the system of allowances.

And it's not just about the John Lewis list. Mr Cameron may have been quick to apologise over expenses, but he has been slow to reveal whether his donor, Lord Ashcroft, is resident for tax purposes in this country. Mr Brown said sorry yesterday - but last year he tried to block the release of MPs' expenses claims under the Freedom of Information Act, and last month he was employing a spin doctor who thought it a good idea to smear his opponents' families. It will take more than an apology and a review to regain public trust.

For more than a decade, the parties have exchanged tit-for-tat allegations over sleaze and now they are paying the price. Only two weeks ago Mr Brown tried to score points by announcing on YouTube a plan to reform the expenses system without consulting Mr Cameron or Nick Clegg. But the dividing line that really matters is between the politicians and the voters. As our poll shows today, 86 per cent of voters think that all the parties are bad as each other in playing the system to their own advantage.

The electorate is saying “a plague on all your houses” to MPs and that is deeply dangerous. Already the British National Party is gaining ground in the run-up to next month's local and European elections by campaigning for the “anti-politics” vote. According to its website, it will spend £390,000 on capitalising on people's anger.

Its candidates present themselves as Westminster outsiders, its leaflets urge people to “punish the pigs” in the House of Commons. And, of course, its real and appalling aim is to make not politics but the country “whiter than white”. Is that really where we want all this to end?

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