Ed Miliband hopes to force David Cameron on to the back foot on Thursday by making it clear that a planned £6,500 pay rise for MPs is unacceptable, and vowing that every Labour candidate will go into the next election promising, if elected, not to have any outside directorship or consultancy.

He also challenged the prime minister to agree on a £5,000 cap on individual, business and union donations to political parties.

The MPs' watchdog, the Independent Parliamentary Standards Authority (IPSA), is expected to propose a £6,500 increase in MPs' current £66,396 salary from 2015 – around 10%. But Ipsa will try to soothe the inevitable public anger by promising that the increase will not cost taxpayers anything because, at the same time, it is ending MPs' £33,000 severance payments, cutting the cost of MPs' pensions and reducing allowances such as the £15-a-head meal subsidy after 7.15pm.

Cameron, aware of the seething private anger among his backbenchers over their frozen pay, has declined to tell his MPs not to take the increase, instead focusing on the need to ensure the package is cost-neutral.

Ipsa will put its recommendations out for consultation on Thursday, and it is likely there will be another review after the election.

Ipsa has reminded MPs it is entirely independent of parliament and they no longer have any statutory influence over their pay.

After a difficult fortnight due to the fallout of the Falkirk vote-rigging scandal, Miliband is determined to show he is sorting out the union funding of his party, although Cameron is not willing to reopen the recently abandoned all-party talks on funding reform.

Miliband needs to show his party and the unions that, whatever the merits of his reforms to the party-union relationship, they will pay political dividends and so are worth the likely internal strife.

At the noisiest prime minister's questions for many years, Miliband noted that hedge funds had donated £25m to the Tories and that the chancellor, 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."

Cameron rejected the £5,000 cap on donations 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.

Tory sources pointed out that the 2011 Kelly review on party funding had suggested that a £10,000 cap on union and individual donations would have meant Tory income falling on average by 76% annually between 2001 and 2010, while Labour would have lost 91% of its income. State funding would be the only way to replace the loss, the Tories said.

Cameron also declined to say whether he supported Miliband's proposed ban on MPs' second jobs, instead citing the example of former cabinet ministers Jack Straw and David Blunkett, who both have outside interests.

The prime minister sought to put the pressure back on Miliband by saying he should help to 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, but at most changes to party rules.

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.

But the GMB union said Miliband's proposal will have far-reaching consequences and could mean the loss of 90% of the party's annual income from the unions.

The government will try to embarrass Labour next week by publishing a bill that will regulate third-party spending, including by unions, during election campaigns. It is claimed in the 2010 election 23 of these third-party campaigners, including the public service union Unison, spent £2.8m – £1m more than that reported in 2005 – and around 9% of the £31.5m spent by political parties on national campaigning.