Boris Johnson has taken aim at Rishi Sunak’s leadership of the Tory party, urging him to slash corporation tax, warning him not to “lose sight” of the commitment to levelling up and heavily criticising his Brexit deal.

The former prime minister sought to play down the prospect of a return to top-level politics but defended his record in office, highlighting a decline in Tory poll ratings since he was ousted from No 10.

He also sought to make light of the partygate scandal which saw him fined for breaking lockdown rules and expressed his irritation at “wokeness” threatening arts and culture.

In his first major speech in the UK since leaving office, Mr Johnson covered several key points:

– He said he would find it “very difficult” to support the Windsor Framework on Northern Ireland’s post-Brexit arrangements as it was “not about the UK taking back control”.

– He indicated that when Rishi Sunak was his chancellor there should have been major tax cuts to attract investors.

– He hit out at April’s planned increase in corporation tax from 19% to 25%, saying the rate should be cut to lower than Ireland’s 12.5%.

We should think not about raising corporation tax but cutting corporation tax to Irish levels or lower and really turbocharging investment to drive levelling up across the whole country

– He said he did not “understand the rationale” behind his partygate fine for “having lunch at my place at the Cabinet table in 10 Downing Street”.

– He warned against losing sight of the levelling-up agenda “that this government was elected to deliver” under him in 2019.

Speaking at an international conference in central London, Mr Johnson said the UK had to show the benefits of Brexit.

“Let’s dare to be different and do things differently. We should dare to be different on the economy. I know that Covid cost a fortune and there was not much we could do about that,” he said.

“But we’re out of it now and there’s no point now in just emulating the high-tax, high-spend, low-growth European model.

“We should think not about raising corporation tax but cutting corporation tax to Irish levels or lower and really turbocharging investment to drive levelling up across the whole country, really showing the world what they wanted to see from 2016 onwards: that we are different now because this is a Brexit government, or this is nothing.

“It is because this is a Brexit government that we got the biggest share of the vote since 1979.”

Mr Johnson said that while he was in No 10 with Mr Sunak as chancellor “what I wish we had done is put a big ‘invest here’ sign over Britain as soon as we were out of Covid, as soon as it was remotely credible … we should have outbid the Irish”.

Mr Johnson claimed that when he was forced out as prime minister, the Tories were “only a handful of points” behind Labour in the polls.

I also care deeply about the agenda that this government was elected to deliver, which was levelling up, and I do not want us to lose sight of that

The average poll shares for the week ending July 10 2022 had the Tories on 30%, 12 points behind Labour on 42%.

Mr Sunak’s Tories are currently on 28%, some 21 points behind Labour by the same measure.

Mr Johnson played down the prospect of a comeback – “I think it very, very unlikely that I will need to do anything big in politics again” – but indicated he would be keeping a close eye on some of the issues he had identified as his political legacy.

That included “supporting and defending Brexit” and championing the cause of Ukraine, he said.

“I also care deeply about the agenda that this government was elected to deliver, which was levelling up, and I do not want us to lose sight of that,” he added.