The government's majority in the Commons has been halved in a key vote on the introduction of identity cards.

Amid heated debate on the proposals in the Commons, MPs backed ministerial calls for the scheme to be linked to passports.

In the first of a series of divisions, MPs voted by 310 to 279 in favour of ensuring that anyone applying for or renewing a passport or other 'specified' document would also have to join the national identity register.

That government majority of 31, down from a notional 64, came as the prime minister was detained in South Africa after aircraft difficulties.

Some 20 Labour MPs voted against their party line.

A second vote saw a majority of 51 in favour of ensuring that all passport applicants are also given identity cards.

Conservatives had warned that the measure would mean "compulsion by the backdoor" and shadow home secretary David Davis said the plan could become a "plastic poll tax".

But Charles Clarke and a range of other senior ministers had urged MPs to back the government's proposals.

The government later won a vote on a compromise option on the funding of the scheme, rejecting by 314 vote to 261, a majority of 53, a Lords bid to delay implementation of the Bill until the costs are firmly established.

Instead an amendment from Labour MP Frank Dobson which would require a report to parliament on the costs every six months was approved.

MPs also backed the government's position on how public services can require ID cards and on the appointment of an independent watchdog.

The Commons earlier passed without a vote a compromise measure ensuring that there would need to be primary legislation before the cards can be made fully compulsory.

The legislation will now be sent back to the Lords, where opposition parties will have to decide whether to risk a clash with the Commons or accept the government's legislation.

Unity

Ahead of the key votes, the prime minister and chancellor also insisted the government's plans were vital to protect national security.

Tony Blair told the BBC: "I think we've won the argument on it.

"People have this idea that there's a problem in civil liberties with people having an identity card and an identity registered today when across all walks of our life this is happening.

"And with the real problems people have today with identity fraud, which is a major, major issue; illegal immigration; organised crime: it's just the sensible thing to do."

Chancellor Gordon Brown also said that "we should do all in our power to prevent you or I having our identity stolen or abused, and to ensure that, for each of us, our identity is secure and protected".

Database

But both the Conservatives and the Liberal Democrats are opposing ministers.

Shadow home affairs minister Edward Garnier said ministers had already been forced to eat "humble pie" on their legislation.

"The problem that the government have faced is that they have, as usual, misused the English language," he told MPs.

"When they say 'voluntary' they mean 'compulsory' and that is why they have got stuck in this particular problem.

"When they say 'may' they mean 'must', when they say 'possibly' they mean 'definitely'."

Garnier also warned that the proposed national identity register would become a "large government computer into which all sorts of private information will go".

Costs

Earlier, during Home Office questions, Clarke has defended the costings of the scheme.

He said the estimated £584m bill was "accurate and likely to go down rather than up".

"Within our current estimates it will affordable to set a charge of £30 for a stand alone identity card issued to British citizens," the home secretary insisted.

Liberal Democrat home affairs spokesman Alistair Carmichael urged Clarke: "Why don't you demonstrate the courage of your own convictions and allow the amendments made in the House of Lords to stand in relation to costs?"

"Surely if this scheme is costed and affordable then you would have no problem demonstrating that before bringing it finally for implementation by this House?" he suggested.