One of Labour’s most prominent donors has described the memory lapses of Ed Miliband and Ed Balls as very embarrassing and raised concerns that such gaffes could reinforce the perception that the party cannot be trusted on the economy.

Lord Noon, a Labour peer who has given hundreds of thousands of pounds to the party, made his intervention after Ed Balls’ inability this week to recall a business donor’s name on the BBC’s Newsnight, and Ed Miliband’s failure to raise the deficit in his conference speech last autumn.

In an interview with the Guardian, Noon said: “I cringed when Ed Balls forgot the name of a business backer and Ed forgot whole paragraphs from his conference speech. It is very embarrassing”.

Gulam Noon, who has given nearly £873,000 to the party, £100,000 of that since Miliband became leader, made his fortune from the manufacture of ready meals and is worth an estimated £75m.

The peer even went so far as to suggest on Thursday that Labour’s leader and the shadow chancellor should “eat almonds every day [said to improve brain power] so they remember things when they are interviewed or speak in public.”

Facebook Twitter Pinterest Ed Balls forgets name of Labour’s small-business taskforce leader on BBC Newsnight

His intervention comes in a week when Labour has come under concerted attack as a succession of business people have condemned the current leadership for being anti-business – while the party itself has only been able to put forward the names of only a handful of supporters in business.

Noon, who is well connected among the donor community, also appears to reflect many other concerns from business people who gave to the party before the 2010 election. Four other donors, speaking to the Guardian anonymously, have said they will not be donating before this election because Ed Miliband and Ed Balls are hostile to business and weak on the economy.

Here’s how Labour should tackle the tax avoiders: an Office of Tax Responsibility | Polly Toynbee Read more

Balls told Newsnight viewers that Labour had won the backing of someone called “Bill” whose surname he could not recall. Miliband did not mention how Labour planned to tackle the deficit during his conference speech in September.

Noon said the mistakes add to a general and inaccurate perception that Labour is not interested in business. “It is a problem,” he said.

Noon also said that he understands why many Labour supporters from the business community have become concerned by the party’s performance over the past year, repeating a complaint he had previously made last October about its plans for a mansion tax.

He said: “Many are very worried about what Labour will do on taxation, it is a very sticky point. They are worried about the 50% income tax rate and don’t know if they can trust Labour, and the mansion tax is a problem too. Business people in London live in houses worth millions but that does not mean that they have the means to pay the tax.

“The party should consider increasing the threshold so they only hit the super-rich. “I just don’t understand why they announced this particular tax because it will lose votes. Sometimes, there is such a thing as being too honest with the electorate.”

Noon has supported Labour since Blair became leader in 1994, when great gains were made in winning over the business community from the Tories. If supportive entrepreneurs are allowed to drift away, they may be difficult to get back, Noon said.

“We should be very careful about alienating the business community. We need to bring them back. If we lose trust, we lose their funds and we rely on the unions alone,” he said.

Lord Noon: ‘At the end of the day, Labour is my party and I want them to win.’ Photograph: Eamonn Mccabe/Eamonn McCabe

Noon, 79, a Mumbai-born Muslim, was created a life peer in 2011. Despite his remarks, he said he will still be supporting Labour and will encourage others to do so. “At the end of the day, Labour is my party and I want them to win. We all need to get behind the current leadership for the election. We should have nothing to fear and everything to play for because it will be close,” he said.

The mansion tax is one of Labour’s main 2015 manifesto commitments, with the proceeds due to be used to fund the recruitment of 38,000 new GPs, nurses, midwives and other NHS professionals.

Under its plans, Labour has said most people who own homes worth between £2m and £3m will pay £250 a month in extra taxes while owners of homes worth “tens of millions” and second home owners would pay much higher rates.

Labour will raise the top rate of income tax back to 50p if it wins the next election, Balls announced last month.

The rate was applied on incomes above £150,000 until 2013, when it was reduced to 45p in the pound by the current government.

The Guardian has contacted ten other individual or corporate donors who gave £5,000 or more to the party in the runup to the 2010 election but who have not given any money since. Four have decided not to give any money this time, three said they did not wish to comment, two said they had not yet decided, while only one said he would be donating before the election.

One of those contacted, who asked not to be named, said: “I live in London, and am very unhappy with the mansion tax. I live in a flat that belonged to my parents and will be asked to pay the mansion tax. I won’t be giving and I won’t be voting for them.”

The row over business perceptions of Labour erupted on Sunday when Stefano Pessina, the acting chief executive of Boots, claimed that the election of a Labour government would be a catastrophe.

Labour began a belated fightback over its relationship with business on Wednesday night. Lord Allen, chair of Global Radio and a long-time Labour supporter, told the Daily Mirror that a Tory government could lead Britain out of Europe.

“One thing that we the business community want is clarity and certainty. If David Cameron spent two years in a debate about whether the UK should be in Europe, there would be neither. We cannot and we should not push Britain towards the EU exit door or put Britain’s future at risk for the sake of party management,” he said.

The former Labour City minister Lord Myners also issued a supportive statement, saying: “Labour is pro-responsible business. It will promote an environment that is consistent with effective competition.”

• This article was amended on 6 February 2015. An earlier version referred to Gulam Noon “voting Labour”, where “supporting Labour” was meant; as a peer, Noon cannot vote in the forthcoming general election.