David Cameron mocked Labour after shadow chancellor Ed Balls failed to remember the full name of a businessman who backed the party.
The shadow chancellor stumbled over the name of the man who chaired Labour's small business taskforce, identifying him only as "Bill".
The memory lapse, which followed a series of attacks on Labour by senior corporate figures, was seized on by Mr Cameron at Prime Minister's questions as he branded Labour "anti-business".
Mr Cameron seized on Mr Balls' gaffe, claiming that the shadow chancellor could only identify "Bill somebody" when he was asked for the name of a business leader who supported Labour.
The Prime Minister joked: "Bill somebody isn't a person, bill somebody is Labour's policy."
In a BBC interview, Mr Balls failed to remember the full name of Bill Thomas, the former HP and EDS executive who is leading Labour's small business taskforce.
Appearing on Newsnight, Mr Balls said he had come from a dinner with "a number" of business figures who backed Labour.
Asked who was there, Mr Balls said: "Bill ... the former chief executive of EDS, whom I was talking to just a few moments ago ... he is a big supporter of ours."
Pressed on his name, he said: "To be honest, his surname has just gone from my head, which is a bit annoying at this time of night."
Mr Balls later said on Twitter: "I know, I know. Bill Thomas, our Small Business Taskforce Chair, will never forgive me. It's an age thing!"
The Prime Minister's quip about Mr Balls came as he traded blows with Ed Miliband in the Commons over taxation and party donors.
The Labour leader accused the Prime Minister of failing to act to close a tax loophole for hedge funds because "too many of his friends would get caught in the net".
He repeatedly pressed the Prime Minister to close the tax loophole which allowed hedge funds to avoid stamp duty on stock-market transactions "costing hundreds of millions of pounds".
The Labour leader claimed: "This is a Prime Minister who won't tackle tax avoidance for the simple reason that too many of his friends would get caught in the net.
"They are the party of Mayfair hedge funds and Monaco tax avoiders and under him you will always know it's one rule for those at the top and another rule for everyone else."
But Mr Cameron shot back: "The most vital election in a generation is coming and people can see the choice: a Labour Party that is anti-enterprise, anti-business and is falling apart under scrutiny; and a Conservative Party turning this country around."
Asked about Mr Balls's memory lapse, shadow work and pensions secretary Rachel Reeves told BBC Radio 4's World At One: "It's something that's happened to all of us, where we forget someone's name when put under pressure.
"Business leaders support all different parties ... Some of them will get involved in the political debate, most of them actually will stay out of it and most business leaders will work with whoever the government is to further their interests and the interests of the economy as a whole, whether that's on skills, on infrastructure investment, on tax or staying part of the EU, which most business leaders are in favour of.
"Business leaders will have different views. They are entitled to express their views, as are workers and everyone else in this country."
Lord Myners, a former chairman of Marks & Spencer and Treasury minister in Gordon Brown's administration, came to Labour's defence, but said the party needed to "talk in a language that business understands".
The peer, now chairman of the London School of Economics, said: " Labour is pro responsible business. It will promote an environment that is consistent with effective competition; it will challenge abusive monopoly behaviour, take steps to promote competition where customers are being disadvantaged - for instance in retail banking - and promoting long term investment rather than casino capitalism that has disadvantaged the UK's economy and social cohesion.
"Responsible businesses take care of their workers and responsible investors step up to the mark in holding companies to account. Voters expect their government to stop tax avoidance by multi-nationals which disadvantage UK businesses which pay their taxes and play their part in supporting society.
"And Labour also provides a certainty over Britain's continuing membership of the European Union and access for British businesses and their employees to the huge market that lies across the Channel.
"Labour needs to get this message over. It needs to talk in a language that business understands. It needs to make clear that a successful economy is a critical condition to a fair and just society."