Theresa May has apologised “unreservedly” for attacking a Labour frontbencher for not being in the Commons, even though she was absent because she was receiving medical treatment.

Speaking at the first Prime Minister’s Questions session of 2018, Ms May referred to comments made by Angela Rayner in a recent interview that Labour’s economic plans for major increases in state spending as a high-risk “shit-or-bust strategy”.

“What would we see from the Labour party?” Ms May asked MPs gathered in the Commons. “We have turned the economy around from the recession that the Labour party left us. What do we know about the Labour party’s economic policies?

“Well we told all about them from the description from the Shadow Education Secretary, who I see is not on her place on the frontbench today.

But it was quickly pointed out to the Prime Minister that Ms Rayner is currently undergoing medical treatment.

“I do apologise,” the Prime Minister replied. “I didn’t realise Mr Speaker. I did not realise the Shadow Education Secretary was herself undergoing medical treatment – I apologise unreservedly.”

“But I do have to say she did describe the economic policies of the Labour party in unparliamentary terms. It did include the word ‘bust’ but she did say the Labour party’s economic policy was high-risk.”

After Ms May apologised for a misguided attack on Ms Rayner, questions were asked of both her spokesman and Jeremy Corbyn’s as to whether Tory whips were made aware of the reason for the Labour frontbencher’s absence.

The PM’s spokesperson said he was unaware whether conversations had been had or not. But Labour sources suggested a “pairing” arrangement had been made – when an MP from one party agrees not to vote, because an MP from the other party cannot make it to the House for legitimate reasons.

Mr Corbyn’s spokesman later said: “As I understand it, they were informed.”

The Conservative whipping operation has been under the spotlight since the defeat on the EU (withdrawal) Bill last year.

During the interview last week with the Spectator, Ms Rayner, tipped as a rising star within the Labour Party, said: “We are in different times, radical times where we need to have a real investment in Britain’s future. Genuinely – I don’t mean that as a slogan, I mean it as an economic strategy.

“It’s a bit of a shit-or-bust strategy, I get that. It’s a high-risk strategy. But all of Britain’s great advancements in the past have been because we’ve had the gumption to take a risk.”

During the session Ms May also reiterated her apology to patients affected by the winter pressures on the health service, including to the tens of thousands who have seen their operations cancelled, as she defended the preparations.