Prime Minister Gordon Brown can wait almost a decade before holding a vote on compulsory identity cards, according to calculations made by the opposition Liberal Democrat party.

That would effectively put the politically difficult decision on ice, with a parliamentary debate on the controversial issue shunted well beyond the next election.

LibDem leader Nick Clegg said delays to the scheme, revealed in leaks last week, had pushed back the day when the government would have to properly address the matter.

"The ID system cannot become fully operational till it is fully compulsory, and that requires additional primary legislation," Clegg, who opposes the cards, told Reuters in a telephone interview.

"Initially (the government) were going to do this in 2010 or 2011. By our latest calculation they probably don't need to do it till 2017 or so."

By then around 40 percent of the population could have already been issued ID cards alongside new passports, on current government plans.

Recent non-committal comments by Brown on whether the cards should become compulsory for all have raised speculation the prime minister is growing cool over the issue.

Conservative Leader David Cameron has written to Brown asking for clarification on compulsion.

But the LibDem calculations suggest Brown has plenty of time to come up with an answer -- with a vote possibly two or three elections away.

The government had originally wanted the cards to be compulsory from the start.

But in a concession ahead of the passing of the Identity Cards Act in 2006 it agreed that they would at first be voluntary.

Compulsion would be introduced at a later stage, and only with new legislation.

Under present plans, British citizens will be put on the new National Identity Register database and issued with an ID card containing an electronic image of their photo and fingerprints when they apply for a passport.

Over 10 years the government calculates that 80 percent of the population would acquire an ID card through passport applications and renewals.

But to force the remaining 20 percent of people to register for ID cards will require the introduction of new legislation making the cards compulsory for all aged 16 and over.

"There is still a major battle in parliament to be had," said Clegg.

"The government is trying to inveigle us all into this by way of an incremental approach.

"They are doing this in a slightly underhand way, phased over a very long time, where they hope people will be bored or inured into accepting it," he said.

"And there is a very significant legislative bridge to cross."

He said there was no reason to suppose that the public would be indifferent to a vote on compulsion in 10 years' time because so many would by then be on the ID card database.

"Just because a large number of people will have been corralled into it does not mean they would find it acceptable or would be immune to the argument that this was an unnecessary thing to do," Clegg said.

LibDem researchers say leaks of Whitehall memos show the rollout of the ID card scheme will be much slower than previously expected, with the public not even due to use the cards to access public services until 2015.

They say the government will not move to compulsion until there is substantial take-up, which will only happen after this broad access to services is in place.

One memo passed to the Conservative party shows that the introduction of ID cards alongside passports has slipped two years from 2010 to 2012.

Using the cards to access public services is not timetabled in the memo to start till 2015.

A Home Office spokesman confirmed the 2010 passport deadline would not be met, but said a new target date was still under discussion.

Both the Liberal Democrats and the Conservatives oppose the introduction of ID cards, saying they are unnecessary, expensive and an intrusion into private life.

The government says the cards will prevent identity theft, tackle illegal immigration, help fight terrorism and make it easier for individuals to access public services.