A bequest to the Tories worth almost £770,000 was among more than £4.8 million received in donations by David Cameron's party in the fourth quarter of 2013.
The money left by Violet Baker led to reports of a family rift, with her sister-in-law claiming the bequest was made out of "spite".
Labour received £3,162,980 in donations over the same period - including £836,875 from trade union Unite - and the Liberal Democrats £1,311,824.
The £769,227 bequest from Ms Baker, who was from Acocks Green, Birmingham, left her family shocked, according to sister-in-law Elsie Clark.
She told the Birmingham Mail earlier this week her family had been raised as staunch Labour supporters and her brother Ray, Mrs Baker's late husband, had saved the money and invested wisely.
Mrs Clark told the newspaper: "I'm thinking 'what did he save all that money for?' And to do what she's done with it - she's done that for spite."
Labour leader Ed Miliband is pressing ahead with plans to reform his party's links with the unions, so that affiliation fees are paid only in relation to members who choose to support the party and sources have admitted there will be a "financial hit" from the change, which is expected to be endorsed at a special conference on March 1.
As well as the money from Unite, the party received donations of £462,235 from Unison and £389,434 from the GMB.
The UK Independence Party received donations worth £270,955 in the last three months of 2013, taking their total for the year to £654,162 - more than double the £314,410 received by Nigel Farage's party in 2012.
The Electoral Commission figures show a total of £9,935,682 was donated to political parties in the final quarter of 2013, along with £1,972,750 in public funds.
Movement for Change - the campaigning organisation initiated by former foreign secretary David Miliband - was fined £5,500 by the Electoral Commission for accepting donations from Lord David Sainsbury, who was not on the electoral register at the time.
The Blairite campaign group Progress was also fined £6,000 for the same breach.
Progress Ltd accepted six donations totalling £390,000 and Movement for Change accepted 11 donations totalling £344,939.60 between December 2011 and April 2013. Lord Sainsbury was registered before and after this period.
The commission's director of party and election finance Peter Horne said: "These organisations failed to check a donor was on the electoral register and have been fined because of it.
"The fines reflect the lengthy amount of time it took both organisations to realise their mistake. We do not think this was a deliberate attempt to evade the rules and both organisations have now put in place new procedures intended to prevent this happening again.
"The law is clear that if political parties, members associations, holders of elected office or candidates are given money by an individual, they must check that the person is on a UK electoral register. It is an offence to keep a donation from someone who is not on a UK electoral register."
Conservative Party chairman Grant Shapps said: "Ed Miliband promised to reform his party's link to the trade unions, but he has caved in to protect his party's funding, giving the unions even more power over his party.
"He has gone back on his promise to stand up to the union bosses, putting Labour's short-term financial interest ahead of securing Britain's future. Rather than cracking down on abuses by trade unions, Miliband has pocketed another £2 million of their money. Nothing has changed, unions will still pick the leader and buy the policies."
Labour analysis of the Electoral Commission figures showed that donors who attended Conservative Leader's Group dinners with David Cameron or other senior Tory figures donated £2.21 million either in their own names or through their companies.
Shadow Cabinet Office minister Michael Dugher said: "David Cameron leads a party with dwindling members but an ever-greater reliance on big money from an increasingly small number of mega-rich donors.
"Cameron's dinner dates with donors are boosting Tory coffers. It's no wonder the Prime Minister won't repeat the bankers' bonus tax when he is bankrolled by bankers and in hock to hedge funds.
"David Cameron is wining and dining the very same millionaires who have been given a tax cut by this Tory-led Government.
"While the Tories are funded by millionaire donors, the biggest share of Labour's income comes from individual members and working people.
"Ed Miliband is changing his party to open up politics to the public. Cameron's Conservatives are now dominated by a rump of millionaire Tory donors whose lives are a world away from ordinary working people.
"Millions are suffering from his cost-of-living crisis but our out-of-touch Prime Minister continues to stands up for just a privileged few."