Emily Thornberry has defended John McDonnell’s definition of rich taxpayers as those earning more than £70,000 a year, but conceded that there are some families in this bracket who may not feel wealthy.

The shadow foreign secretary said she would only discuss details of Labour’s tax plan after the party’s manifesto was published, but added that those on £70,000 were earning well above the average income.

On Wednesday, McDonnell, the shadow chancellor, told the BBC he would set out a “fair taxation system” and be “looking to the corporations and to the rich to pay their share”.

When pressed to define who the rich are, McDonnell said: “The rich will be above £70,000 to £80,000 a year and that’s roughly defined as what people feel is an earning whereby people feel they can pay more.”

The comments led to a debate on the airwaves and social media about whether someone earning a salary of £70,000 should be considered rich, given the different cost of living in various parts of the country, especially London.

Just over 5% of taxpayers earned at least this amount in 2014-15. Those paid between £15,000 and £50,000 made up two-thirds of UK taxpayers.

Speaking on Thursday, Thornberry said wealth would be relative to other factors. “I think there are many people on £70,000 who may well feel that their circumstances are such that they are not rich,” she told BBC Radio 4’s Today programme.

“Many people feel that the elite has not been taken on. People who don’t think the rules apply to them. People who feel they can keep their wealth offshore and avoid paying taxes.

“There has begun to be a view around that ordinary people are the only ones who should be paying taxes and it’s about time that stopped. It’s been glacial and we can see there are people who take the mickey, and it’s not fair.”

Launching Labour’s election campaign later on Thursday in Westminster, Jeremy Corbyn will pledge to “overturn the rigged system” – singling out “tax-avoiding multinational companies”.

The Labour leader will deny that the election is “a foregone conclusion” and say he will refuse to play by the normal rules of politics.

“They say I don’t play by the rules – their rules,” Corbyn will say. “We can’t win, they say, because we don’t play their game. They’re quite right I don’t. And a Labour government elected on 8 June won’t play by their rules.”

Corbyn will deliver a message on the state of the economy, living standards, schools and the NHS. “When we win, it’s the people – not the powerful – who win,” he will say. “The nurse, the teacher, the small trader, the carer, the builder, the office worker win. We all win.”