Ed Miliband is facing fresh pressure after it emerged that Labour has been lagging well behind the Tories in raising money for the general election.
Figures released by the Electoral Commission showed the party received £4.4 million in significant donations in the first quarter of the year.
That included more than £2.5 million from unions Unite and Usdaw.
However, the Conservatives raked in £6.6 million.
The details were published as Mr Miliband introduced American electoral guru David Axelrod to the shadow cabinet.
The leadership is hoping that the adviser to Barack Obama can boost Mr Miliband's personal ratings and halt the apparent Tory poll revival.
A £1.5 million gift from hedge fund mogul Sir Michael Hintze helped swell Conservative coffers, along with £257,500 each from party co-treasurers James Lupton and Michael Farmer.
Labour received £1.8 million from Unite, thought to be among the union's biggest quarterly donations, despite suggestions that it would be cutting funding.
USDAW gave the party £692,000, and Unison handed over £420,049.
Even with a £1.8 million contribution of "short money" from the public purse - designed to offset the benefits of civil service support to parties in government - Labour still had £400,000 less in donations than the Tories.
The Liberal Democrats were given £1.07 million, and Ukip - favourites to win next week's European elections - £493,000.
The Scottish National Party (SNP) added £1.05 million to funds as it fights for a Yes vote in the independence referendum north of the border.
Mr Axelrod was hired last month on a reported six-figure salary to provide advice during what will be a fiercely contested election campaign.
The challenge was underlined as he arrived in London yesterday, with more evidence in polls that David Cameron's Conservatives are benefiting from the turnaround in Britain's economic fortunes.
The political consultant was Mr Obama's campaign adviser during his successful run for the presidency in 2008 and senior strategist for the president's re-election campaign in 2012.
Labour has said he will be an integral part of its team despite being based in the US, and he will work closely with campaign co-ordinator Douglas Alexander and the party's senior pollster Stan Greenberg.
Mr Axelrod dined with Mr Miliband last night, and is expected to hold detailed discussions with Mr Alexander this evening before leaving the country.
Conservative Party chairman Grant Shapps said 75% of Labour's donations in the quarter had come from trade unions.
"Ed Miliband said he'd stand up to his union paymasters like Len McCluskey - but he's failed and broken his promise. Ed Miliband is too weak to stand up to his union bosses and that means he's too weak to stand up for Britain," he said.
"Union bosses are still in the driving seat at Labour HQ. They are holding the purse strings, selecting the candidates and picking the policies.
"We all know what this means - more borrowing, more spending and more taxes hitting families across the country. It's the same old Labour Party."
Labour frontbencher Michael Dugher said: "Hard-working people are worse off under David Cameron, but his party is better off.
"David Cameron's party is increasingly bankrolled by a small number of millionaire bankers and dinner donors. It's no wonder his government stands up for a privileged few over hard-working families.
"Half of the many millions rolling in to Tory coffers during the last quarter came from a wealthy elite who have exclusive access to the top table.
"These are the same millionaires who have been given a tax cut by the same Tory Government which is failing to address the cost-of-living crisis facing families.
"When the Tories are reliant on so few, the many are being overlooked."
Labour said its total income for the first three months of 2014, including small donations and membership fees not recorded by the Electoral Commission, was £8.2 million.