Analysis By Nick Assinder Political correspondent, BBC News website

One of Gordon Brown's "big ideas" when attempting to show he would be a new broom in No 10 was to float the idea of creating a written UK constitution.

Mr Brown spoke of written constitution

The message was that such a document would underpin citizens' rights and civil liberties in a rapidly-changing world, while co-incidentally showing voters the new, Scottish, PM was committed to the idea of "Britishness".

When it came to legislation, however, Justice Secretary Jack Straw adopted a more piecemeal approach, with plans to give MPs a vote on going to war, limiting prime ministerial prerogative and so on.

The creation of a Bill of Rights and Responsibilities was left to another day amid suspicions the idea of a full-blown written constitution had been quietly dropped.

But now, Mr Straw has re-opened the debate by suggesting the Bill he is currently working on - but which is not expect to be legally enforceable - might provide the basis for a future written constitution.

Civic duty

Once again, the message the government wants to put out is that it is determined to codify a citizen's rights while also making a clearer statement of what constitutes an individual's civic duty - a new form of social contract.

"As we become a more heterogeneous society it's a good idea to put in place these more explicit building blocks of a constitution and then over time to aim for a single formal document," he told the BBC Radio 4 Today programme.

Mr Straw said laws could be 20 years away

But this is a minefield - precisely why there has never been a written British constitution before now.

Currently, the constitution is comprised of a number of documents - none of them comprehensive - including the 1215 Magna Carta, the 1689 Bill of Rights, the 1701 Act of Settlement and the Parliament Acts.

Most recently, the controversial Human Rights Act - brought into force by Mr Straw when Home Secretary - has been added to that library and it remains unclear how any new laws would impact on that - if at all.

Big Brother

Mr Straw probably best summed up the current situation, declaring the constitution "exists in hearts and minds as much as it does in law. But most people might struggle to put their finger on where their rights are".

To critics of his proposals, that is the whole point. The moment a government attempts to spell out what a citizen's rights are it equally sets out in a concrete, legal framework, precisely what is not a right.

And who, they ask, will ultimately decide what those rights are - parliament or the courts?

Supporters, however, believe citizens deserve and need such a document, partly as a defence against any future governments that might attempt to limit their rights.

The fact that Mr Straw has revived this idea at the same time the government is being accused of marching Britain into a surveillance society, or Big Brother state, is seen by some as more than coincidence.

And there are already questions being raised over why Mr Straw believes, judging by his Today interview, that such a constitution might take up to 20 years to become a reality.