The Lords has, of course, already changed substantially, notably via the introduction of life peers and the removal of most of the hereditaries in 1999 — though it still plays an important role in revising legislation and in scrutiny. But this has not resolved the question of whether the Lords should be predominantly appointed, as now, or have a large elected element.

The 1911 Parliament Act, the first to limit the powers of the Lords, remains unfinished business. Reform plans floundered in the late 1940s, 1960s and 1970s, mainly because of opposition in the Commons.

In a weighty lecture last night to the Constitution Unit, the independent academic monitor, Mr Straw sought a way out of this dilemma. The status quo, the preferred view of a majority of both Tory and Labour peers, would not do, he argued. Reforms to make the Lords “more representative, legitimate and accessible to the public” could not be left out of the wider programme of parliamentary reform.

Mr Straw set out key principles: a reformed Lords must not be a rival to, or a replica of, the Commons; it must be more representative; no single party must have an absolute majority; and some crossbenchers must be retained.

Few would dispute these goals, but how do you reconcile election with not reproducing clones of Commons politicians? And can you formalise the powers of the Lords at the same time as making the chamber more legitimate.

As Professor Vernon Bogdanor wrote in a letter to The Times yesterday, an elected element will inevitably increase the power and influence of the Lords, yet again stiffening the resistance of MPs to the creation of a chamber that might challenge them.

Mr Straw’s “best guess is that a consensus is most likely to be found in a balanced, hybrid House, with the change phased in over several Parliaments”. But “deadlock again will be very easy to achieve”. Even if, in contrast to the rejection of all options in February 2003, the Commons does express a preference in a vote early in the new year, the Government will have to be tactical: hence, the emphasis on phasing to avoid upsetting existing peers.

But the details matter, as Mr Straw discovered when he faced a rough ride from Labour peers last week. Tony Blair had better be well briefed when he addresses them this evening. The risk for the Government is that, once Lords reform gets going, the whole legislative programme will be disrupted.

This debate is occurring against the background of a more active, and influential, Parliament, not a weaker one. Mr Straw demolishes that improbable duo, Baroness (Helena) Kennedy (who chaired the Power Inquiry) and Simon Heffer, in their parallel claims that MPs and peers are a supine bunch cowed by the executive, patronage and the whips.

Never have MPs and peers been more rebellious and assertive. That is why the debate over Lords reform matters.

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