Philip Hammond has been forced to defend his 2% hike on national insurance for self-employed people in the UK - and cited Brexit as the reason for what many believe to be a broken election promise.
Conservative MPs are encouraging the Chancellor to think over his plans to raise national insurance by £240 a year for 2.5 million self-employed workers.
The tabloid press has been fiercely critical of Mr Hammond's decision, while Labour claim he has breached the Tories' 2015 election manifesto pledge to freeze NI contributions for the duration of this Parliament.
In an interview with ITV's Good Morning Britain, Mr Hammond maintained the circumstances in the UK had changed since the 2016 EU referendum, and insisted the self-employed have to make a "fair contribution."
He said: "Britain's circumstances have moved on. We're now facing the challenge of leaving the European Union.
"We're now trying to build a global Britain to exploit the opportunities this country can enjoy, and we need to invest to do this.
"I'm asking the self-employed to pay a little bit more National Insurance in order to make a fair contribution for the services they receive from Government."