It seems that no constitutional change can be suggested by government without instantly being termed a Maastricht moment.

Today, it is House of Lords reform [2]. Last night, at a hostile meeting of the 1922 Committee – the influential group of Conservative backbenchers – MPs warned that they would revolt.  The Daily Mail [3] quotes one MP saying that it would "make Maastricht look like a tea party". House of Lords reform – making the upper chamber 80 per cent elected [4] with 15 year terms – is set to be the centrepiece of the Queen’s Speech.

Reportedly, more than 90 MPs signalled their unhappiness with the bill at last night’s meeting, with just one backbencher, Gavin Barwell, speaking in favour of the policy. Seven Parliamentary Private Secretaries (PPS) said they would resign from their positions.

There are several reasons why this rebellion is important. House of Lords reform is an important coalition issue. David Cameron is believed to have given Nick Clegg a personal assurance that he will make sure the bill goes through. Neither of them will want a re-run of the bitter battle over the AV referendum.

Indeed, it is this very fact that is enraging many Tory MPs, who are angry that a serious constitutional change that could cause political deadlock is being waved through to appease the junior coalition partner. Downing Street has reiterated that the Conservative manifesto committed the party to Lords reform:

We will work to build a consensus for a mainly elected second chamber to replace the current House of Lords, recognising that an efficient and effective second chamber should play an important role in our democracy and requires both legitimacy and public confidence.

(Over at ConservativeHome, Paul Goodman [5] disputes this commitment).

If all those who said they would rebel carry through their threat, the backlash could surpass that seen over Europe, when 81 Tory rebels defied the party whip. That could place Cameron in the uncomfortable position of relying on Labour to get the bill through, which would further alienate Conservative members. It is also a high-risk strategy: if Ed Miliband’s party decides not to play ball, the government could have an embarrassing defeat on his hands.

What is particularly notable about this rebellion is that it includes some of Cameron’s most loyal backbenchers. Loyalist MPs Jesse Norman and Nadhim Zahawi, both close to George Osborne, have been leading the backlash against the bill. This is by no means a protest confined to the dissenting right-wing of the party.

Cameron has form on facing down his critics in the party, but this episode presents even more of a political headache. Reform of the upper chamber has long triggered intractable arguments. The political stakes are high as the government attempts to win it once and for all.