The Lisbon treaty should be amended to safeguard the rights of the Westminster Parliament. This is as much a pro-European as a sceptic point. So far, the treaty has been debated in extreme terms: it is either just a tidying-up exercise or a significant transfer of powers. These arguments will be rehearsed at length in the Lords today with more than 70 peers due to speak on the Bill’s second reading. Whether or not to hold a referendum will be the big drama in a couple of months, though the splits within the three main parties will be both larger and very different from those in the Commons. But it is not all about a referendum. Other important issues have been highlighted in reports by two Lords committees, on the European Union and the constitution. These focus on the Government’s central claim, much disputed by the sceptics, that there is nothing to worry about. On this view, the treaty is about adjusting to a union of 27 and neither represents a fundamental change in Britain’s relationship with the EU, nor will it permit further shifts by the back door.

The Lords Constitution Committee concludes that the treaty would make no alteration to the relationship between the principles of the primacy of EU law and parliamentary sovereignty, and would also have no constitutional implications for UK citizenship. But there are ambiguities, notably over the Charter of Fundamental Rights.

The key point is not the change in status from a political document to one having the force of a treaty, since the declaratory rights are already recognised in law by the courts. What matters more is that the Government has claimed that the protocols obtained by Britain, and Poland, offer protection against attempts to create new rights and extend competences into new areas. But the committee, and its legal advisers, such as Professor Alan Dashwood, from Cambridge, argue that the protocol interprets the application of the charter, and is not an opt-out. The other big area of controversy is about further alterations to EU treaties. Under existing British law, originally passed in 1978 when David (now Lord) Owen was Foreign Secretary, no treaty increasing the powers of the European Parliament can be ratified unless approved by an Act of Parliament. But clause five of the new Bill creates new requirements for parliamentary approval of changes, including internal policies and actions.

Passerelles, or bridges, in the treaties allow heads of government to agree changes without formal revision of the treaties. This will still involve votes in Parliament, but the Lords committee wants the Bill to be strengthened, by requiring written statements on, and sufficient time to consider, any treaty changes under the passerelle provisions. The peers also recommend an amendment to require the Government to obtain approval from Parliament before using opt-ins or opt-outs in areas such as security and justice.

This is a fluid area depending on rulings by the European Court of Justice. It is here that the Government’s claims will be tested. But the amendments urged by the peers could provide additional reassurance and should be accepted by ministers.

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