Stephen Flynn told Rishi Sunak to "grow up" after the Prime Minister joked about SNP MSPs sending flowers to Nicola Sturgeon.
The SNP Westminster leader had asked Sunak during Prime Minister's Questions if he thought mortgage rate rises were going to cost his party the next election.
The Tory PM mocked Flynn with his reply, asking if he signed the card that was sent to Sturgeon along with the flowers.
Flynn then said that Sunak "needs to grow up" and address the "elephant in the chamber of Brexit."
SNP MSPs sent flowers to former First Minister Sturgeon as a mark of sympathy following her arrest.
Economists have warned that mortgage rates are set to rise further earlier this week.
Flynn claimed Sunak had previously “warned of the perils of mortgage rate rises” during Prime Minister's Questions.
He said: “During his ill-fated leadership bid late last summer, the Prime Minister warned of the perils of mortgage rate rises, indeed he stated and I quote, ‘it’s going to tip millions of people into misery and it’s going to mean we have absolutely no chance of winning the next election’.
“Given that mortgage rates continue to rise, does he still agree with his own electoral analysis?”
Sunak replied: “Which is absolutely why our economic policy sets as our number one priority to reduce inflation and thereby bring interest rates – which, by the way Mr Speaker, have also risen in pretty much every developed economy around the world – more so in places like America and New Zealand, similar in other countries as well, like Australia.
“But it is important in order to reduce inflation that you do need to have control over borrowing, which is why we unlike the party opposite are disciplined with regard to the public finances.
“But these are the issues that we should be focused on. I saw that the SNP yesterday, Mr Speaker, had a meeting to discuss their future, but the only thing they managed to decide was that they should send Nicola Sturgeon some flowers. So can he tell us, did he sign the card?”
Flynn said: "Respectfully I think the Prime Minister needs to grow up.
“There is an elephant in this here chamber when it comes to the dire economic circumstances facing the UK, and that’s Brexit.
“The Tory benches they don’t want to accept it and the Labour Party, they don’t want to talk about it, but whether it’s food prices, energy prices or indeed mortgage prices, households in Scotland are being shafted by Brexit. So can I ask the Prime Minister will he apologise for the cycle of misery that Westminster has caused?
Sunak replied: “While his party leader calls Nicola Sturgeon the most impressive politician in Europe, we are getting on delivering for the people of Scotland.
“Paying half of their energy bills, making sure that pensions rise, making sure that there’s direct support for those who need it with the cost of living and crucially ensuring that we secure over 200,000 jobs by supporting Scotland’s North Sea oil and gas industry, something opposed by his party.”
When asked if it was appropriate for Sunak to mock the SNP when asked a question about mortgage rate rises, the Prime Minister's official spokesman said: "I won't get into engaging with political commentary during parliamentary questions, but obviously the Prime Minister has set out his position very clearly."
When asked if he needed to grow up, Sunak's political spokesperson said: "He doesn't engage with name-calling."
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