David Cameron has rejected a proposal from Ed Miliband to impose a £5,000 cap on individual donations to political parties as the UK Labour leader sought to turn the tables on the British prime minister in the row over trade unions.

In some of the rowdiest scenes in parliament since the last general election, Labour MPs shouted "weak" after Miliband opened the weekly session of prime minister's questions by revealing that hedge funds have donated £25m to the Tories.

Miliband, who was unsettled after the prime minister taunted him last week over the influence of the Unite union, sought to regain the initiative by asking Cameron how much hedge funds have donated to the Conservative party.

The prime minister declined to answer, prompting Miliband to reveal that they have donated £25m to the Tories and that George Osborne granted hedge funds a £145m tax cut in the budget.

The Labour leader said: "This is a man owned by a few millionaires at the top of society and everyone knows it. Here's the difference between him and me … I want party funding reform; he doesn't. I am proud that we have links with ordinary working people. He's bankrolled by a few millionaires – the party of the people, the party of privilege."

Buoyed by Labour MPs shouting at the prime minister to answer the questions, Miliband asked the prime minister whether he would agree to a £5,000 limit on donations from trade unions, businesses and individuals "as part of a fundamental reform" of how Britain's political parties are funded.

The prime minister rejected a £5,000 cap on the grounds that taxpayers would have to make up the difference in funding. "I don't see why the result of a trade union scandal should be every taxpayer in the country paying for Labour," Cameron said:

Miliband said: "So there we have the truth. He is ducking funding reform. He doesn't want it to happen." Labour sources insisted that they were not proposing state funding of political parties.

The Labour leader opened another front when he asked the prime minister whether he would support his proposal to prevent MPs from taking on new paid directorships and consultancies after the next election.

Cameron declined to answer the question as he cited the example of former cabinet ministers Jack Straw and David Blunkett, who have outside interests.

Miliband said: "He has got to do a lot better than that. He has got to answer the question on second jobs."

The prime minister sought to maintain the pressure on Miliband by saying he should help amend a government bill next week to implement the Labour leader's plan to require individual trade unionists to opt into affiliating to the party. This was dismissed by Labour as a stunt because Miliband's proposal does not require legislation.

Cameron said the Miliband reform plans, outlined in a speech on Tuesday, had not diminished the dominant role of trade union leaders in the Labour movement.

The prime minister said: "What has actually changed since yesterday? Will the unions still have the biggest vote at the conference? Yes. Will they still be able to determine the party's policies? Yes. Will they still have the decisive vote in voting for the Labour leader? Yes. That is the facts. They own you lock, stock and block vote."