The pound's decline has prompted renewed calls for Britain to join the euro. The debate is important, but the euro remains wrong for Britain. The issue arises now not because of inherent problems with a system of monetary independence, but because UK policymakers have made mistakes. Those errors would not have been avoided, might have been aggravated and will not be resolved by adopting the euro.

There is force in the charge made by the Conservatives that the economic crisis is being invoked to advance narrow political ends. Even the greatest financial crisis in a generation will eventually pass; but the constitutional change marked by the abolition of the pound would be irrevocable. The euro debate should be immunised from short-run questions of the level of sterling.

Considering the longer term, sceptics must, however, acknowledge that the euro has not only survived the past decade but has become a stable reserve currency. That is a gain for its member states, especially those that have a history of inflationary problems. But in adopting the euro, these countries have given up the ability to use the exchange rate to offset an economic shock, and are subject to a single interest rate that may not be suitable for their particular stage of the business cycle. The main macroeconomic question for the UK is whether the benefit would justify that cost. It would not.

The pound's weakness is admittedly not the gain that some commentators have suggested. It provides a boost to export earnings, but this is hardly substantial when global demand for traded goods is collapsing. Sterling's depreciation indicates, among other things, that international investors are concerned about the deterioration of the UK's public finances. Poor fiscal management, excessively easy monetary policy and failures in financial regulation have made the UK particularly vulnerable in the global downturn.

The remedy to these errors is not to transfer monetary decisions to the European Central Bank. It is to restore the credibility of the UK's policy regime by better decisions and by institutional reform (for example, amplifying the remit of the Bank of England to take account of asset price inflation, and restoring its responsibility for banking supervision). These and other remedial measures can be taken independently, and might be more difficult as part of the eurozone. For example, Britain's housing bubble had its counterparts in the Irish Republic and Spain, which are now unable to set interest rates independently to cope with the damage.

The strongest objection to joining the eurozone, however, is political. EMU involves giving up monetary independence while retaining control over public finances. There is an unresolved tension in that arrangement. National governments are free not to comply with EMU's fiscal rules, but they would thereby undermine an arrangement designed to establish price stability.

This is where the constant pressure for centralising power stems from. It is part of the logic of the integrationist project, and is acknowledged in the statements of European leaders that monetary union is a stage towards political union. Political union is neither desirable nor democratic. British foreign and defence policy, welfare provision and economic priorities are the prerogative of Britain's Parliament, and must remain so.