Tory plans to put a cap on migrants coming into the UK would tie industry's hands, the head of Britain's top business association has said.
It is impossible to predict the needs of business, says the CBI
Confederation of British Industry director general Sir Digby Jones said a limit would damage flexibility as skills needs constantly changed.
Tony Blair accused the Tories of trying to exploit people's fears, while the Lib Dems said they pandered to racism.
Michael Howard said firm controls were needed to preserve community relations.
The Conservatives plan to impose fixed limits on the number of immigrants coming in Britain to work, as well as cap the number of refugees granted asylum.
Sir Digby said plans to impose any limit on immigration would leave firms unable to recruit the skilled workers that they need.
He told BBC Radio 4's The World Tonight: "If we have a cap of any sort it will tie businesses' hands from the flexibility which is, after all, the hallmark of the British labour market. So we are not in favour of a cap.
"To say we are going to cap them at a certain figure, then, frankly, unless that figure was so enormous we would say 'you are tying our hands when you don't need to'."
Sir Digby said every 1% increase in immigration brought a 1.5% increase in national wealth and that 97% of immigrants found work straight away.
"If it was not for immigrant labour, especially in leisure, in tourism, in agriculture, in construction, then frankly many of our businesses would not have the workers we need," he added.
Shadow home secretary David Davis defended the plans, saying: "We believe immigration plays a part in creating a competitive and dynamic modern economy, but we also think that the system needs to be managed.
"Under our Australian-style quota system we would consult with organisations like the CBI to decide how many people Britain needs."
In an interview with the BBC's Jeremy Paxman, Mr Howard said: "There is a clear limit to the amount of immigration this country can absorb - and it is in the interests of the minorities themselves to maintain a strict control."
He also argued that Britain should take note of the way the Irish Republic has massively reduced the number of work permits it gives out.
Mr Blair earlier accused the Tories of exploiting the issue and said it was vital immigration and asylum were not used to divide people or turn British tolerance on its head
He attacked the way the Tories had linked immigration with racism in campaign posters.
"It is an attempt deliberately to exploit people's fears, to suggest that for reasons of political correctness, those in power don't dare deal with the issue," he said.
New controls, including a points system for economic migrants, more detentions and tagging for asylum seekers, were also planned by Labour.
Trade and Industry Secretary Patricia Hewitt then said she felt the Tories were pandering to racism without offering any solutions to the issue.
Lib Dem president Simon Hughes said he thought the Tories were "pandering to racism".
"I am quite unequivocal about it...I think it is despicable".
Lib Dem leader Charles Kennedy said these comments were "more trenchant" than he wanted to be.
But he added: "When a senior politician starts talking about immigration in the way that Michael is choosing to, we know that there is a strata of opinion in our society that will take that as a bit of a green light to behave and talk in an irresponsible way. We mustn't do that.
The responsible thing to do was to call for controlled immigration without stirring the issue up, he added.
The Lib Dems would create an independent commission to set quotas for economic migrants based on the needs of different sectors of the economy.