Mr Clarke is expected to make an announcement on Monday

The prime minister said immigration would be tightened so only those whose skills the UK needs get work permits.

But he rejected the Tories' call for a quota, saying economic migrant numbers should reflect the UK's needs.

The Lib Dems say they will look at the plans, but Tory Liam Fox said his party offered a "clear choice" on the issue.

Home Secretary Charles Clarke is due to announce plans on Monday for a points system similar to Australia's for people who want to move to the UK.

The public are worried about this, they are worried rightly, because there are abuses of the immigration and asylum system Prime Minister Tony Blair Click here to read a transcript of the interview Hear the interview

Speaking to BBC Radio 4's The Westminster Hour, Mr Blair said: "I don't think you should have an arbitrary figure [on economic migrants] - I don't agree with the Conservative's proposal for a quota.

"We should cut the number or increase it depending on the country's needs."

But he said the system could be "toughened up" to make certain only those the country "really needs" get work permits.

On abuses of the asylum and immigration, he said all systems around the world were subject to abuse.

Mr Clarke is expected to make an announcement on Monday

"The public are worried about this, they are worried rightly, because there are abuses of the immigration and asylum system," he said.

But he would not put a figure on the number of people who remain in the UK after their asylum claim had been rejected.

And he said Tory proposals to cap the number of asylum seekers and process all claims abroad would not work.

"We will be putting forward policies that tackle the abuses, that have strict controls, that work," he said.

The jury is still out on the Home Office's ability to deliver a fair and efficient asylum system Mark Oaten, Lib Dems

"We will not be... pretending there is some simple easy way of processing Britain's asylum seekers in some other country, because no such other country exists."

On Monday Mr Clarke is expected to announce a five-year blueprint for immigration and asylum.

He told BBC One's Breakfast with Frost that economic migration was "of great value" but needed proper policing.

Liberal Democrat home affairs spokesman Mark Oaten said Labour was right to reject the Tories' idea of quotas on asylum, but it was yet to be seen if it could deliver "a fair and efficient asylum system."

But Conservative Party Co-Chairman Liam Fox said Britons had a choice between a Labour government that had "done nothing for eight years" and a Tory one that would impose quotas.

Tory leader Michael Howard has said Britain should take its fair share of the world's "genuine refugees".

But he claims the current asylum system is being abused - and with it Britain's generosity.

The Refugee Council said Mr Howard's proposals would mean there would be no safe haven in the UK.