The response of the government to a parliamentary committee report on constituency boundaries, published this week, ran to 16 pages but might be summarised in two words, the second of which is “off” and the first unsuitable for print. The document was released with maximum discretion, yet it contains a big choice the government has made, with repercussions for parliament and the next election. It is the decision to reject completely MPs’ advice to amend rules, passed in 2011, for drawing new boundaries. That means the total number of seats will fall by 50 and the remaining 600 will be adjusted to reflect what ministers say is a more equal share of voters.

This is justified by David Cameron on the grounds that reducing the number of MPs cuts “the cost of politics”. The Conservatives had planned to introduce the changes in time for last May’s election, but the timetable was frozen by the Liberal Democrats in retaliation for Mr Cameron’s failure to reform the Lords. Eagerness to unfreeze the process – getting new boundaries drawn without recourse to new legislation – is the reason the government has brushed aside the cross-party political and constitutional reform committee’s view. MPs had queried the method of change, urging ministers to draft something fairer; and to be quick about it. That was March 2015.

But Oliver Letwin’s department did not bother replying within the usual timeframe of a few weeks, waiting instead until the committee’s recommended deadlines for action had passed. So it will be according to the 2011 rules, regardless of flaws identified by the committee, that the new boundaries must now be presented to parliament by October 2018

There is a theoretical justification for the changes. The UK’s lower chamber is big compared with similar democracies but not outlandishly so. (The Lords is ludicrously bloated.) Population flux means boundaries always need tweaking. But the Tories are also convinced that the old map contains a pro-Labour bias. The redrawn version could have the reverse effect, giving the Tories a massive head-start in 2020.

But most cynical of all is the decision to base the “equalisation” of constituencies on a version of the electoral register that potentially omits hundreds of thousands of people, especially students and recent migrants. MPs had advised that some consideration be given to population data, balancing seats to an average of the whole electorate, not just registered voters. Instead, the government has quietly truncated a transitional period that would have carried many voters from the old household-based register on to the new individually registered roll. This obscure technical change looks like a deliberate, aggressive manoeuvre to make sure the new constituencies are as Tory-friendly as possible.

MPs are hardly neutral judges of all this. Reducing their number by 50 triggers a nasty round of musical chairs in all parties as incumbents seek selection for fewer seats. That is not in itself a reason to avoid reform. But as the committee report made clear, the case for these particular reforms is unmade. It would in any event have been unwise for ministers to dismiss those reservations. To have done so with such high-handed contempt is an affront to parliament and a symptom of unchecked arrogance that leads inevitably to bad government.