David Cameron has been accused of an unjustifiable bid to “buy the general election” as it emerged that ministers have quietly slipped through an unprecedented hike in the amount that parties can spend during the campaign.

Before this week’s official start to the runup to the 2015 general election, the Observer can reveal that the Conservatives have ignored Electoral Commission recommendations and secured a 23% increase in spending. With the Tories having amassed a £78m war chest over the past four years, they can now funnel huge amounts of cash into key seats.

The change to the law on candidates’ election spending, passed without parliamentary debate, was made despite a direct warning by the commission against such “excessive spending to prevent the perception of undue influence over the outcome of the election”.

Lucy Powell MP, Ed Miliband’s key general election strategist, admitted that Labour “can’t match the depths of Tory pockets” and accused the prime minister of seeking to “buy an election they don’t deserve to win”.

Writing on this newspaper’s website, she says: “With only a record of failure to run on, David Cameron’s campaign is reliant on smear, fear and fat cats’ cheque books. This is a party flush with big money backers but without the empathy or ideas the country needs, so they are rigging the rules of our democracy in their favour.

“When he was first leader of the opposition, David Cameron said he wanted to take the big money out of politics. He promised to address the ‘big donor culture’, arguing that we should, ‘cut what is spent on a general election’. Yet he has now cynically changed his tune. Desperate to hang on to power, the Tories have quietly changed the rules to allow them to spend big in the runup to the election. The changes would allow them to spend millions more than they’re presently allowed, paving the way for Tory propaganda to flood constituencies.”

Under the new limits, the total amount the candidates of each political party can spend has increased from £26.5m to £32.7m. In March, the Electoral Commission recommended there should be no such increase in spending limits for candidates over the so-called “long campaign” period between 19 December and general election day on 7 May.

It suggested there should only be an increase of £2.9m on spending during the “short campaign”, which starts 25 days before the election, to take account of inflation and some changes in the length of the election campaign.

At the time the commission dismissed the Conservative party’s submission that there should be a bigger rise because “new developments in campaigning technology are not making politics cheaper”. It has now emerged that the government went ahead in the summer and slipped through increases twice those recommended by the commission.

The Observer has learned that ministers changed the law through a statutory instrument, the terms of which were not debated in the Commons, and which is more usually a vehicle for consensual changes in the law. A Labour source said that the move had not been spotted by them at the time and so they missed the chance to force a vote in the Commons.

The Conservatives, who are paying the Australian strategist Lynton Crosby £500,000 to mastermind their campaign, are in a strong position to take advantage of their changes because the donations they receive vastly outstrip that of the other parties.

This summer alone, the Tories accepted more than double the £3.1m that Miliband’s party received.

In the past four years the Tories have raised £78,010,807, with £21,072,508 (27%) coming from hedge fund donors. George Osborne’s 2013 budget abolished stamp duty reserve tax on funds, a £145m giveaway to hedge funds.

Since the last general election, the Tories have also received £6m from secretive unincorporated associations, such as dinner clubs, which do not need to declare the identities of their members. This year the Carlton Club political committee, and the United and Cecil club have accelerated their donations to marginal seats, according to electoral commission figures. In the last quarter, 34 Conservative associations, all in marginal seats, received between £2,500 and £7,500 from the two clubs, totalling £116,600.

The United and Cecil Club, whose honorary secretary is a former tobacco lobbyist, has made 59 donations to key seats since 2010. A donor who gives through a members’ club is only named by the commission if their gift is in excess of £7,500.

A Cabinet Office spokesman said: “It is crucial in a robust democracy that candidates are provided with a reasonable level of expenditure which would enable them to communicate properly with voters. An MP who wanted a debate on the rise could have called for one in the usual way.”