Prime Minister Boris Johnson has insisted the “golden rule” is to “focus on what we are doing” after revealing he is planning to be in office into the 2030s.

He admitted he has not “had time” to reflect on the biggest regret of his premiership so far, but claimed the Government’s achievements have been “remarkable”.

It comes as pressure has been mounting on the Conservative Party leader from across the political divide following the Tories’ stinging by-election defeats in Wakefield and Tiverton and Honiton.

Mr Johnson said during a trip to Rwanda this weekend that he is “thinking actively” about fighting the next two general elections to become the longest-serving post-war leader.

Commonwealth Heads of Government MeetingPrime Minister Boris Johnson recently travelled to Rwanda, and his political troubles travelled with him (Dan Kitwood/PA)

Asked at the G7 summit in Germany on Sunday if his ambitions are delusional, Mr Johnson said: “What I’m saying is this is a Government that is getting on with delivering for the people of this country and we’ve got a huge amount to do.”

He said the “golden rule” is to “focus on what we are doing” – to address the cost of living, the “massive” plan for a stronger economy, and “making sure that the UK continues to offer the kind of leadership around the world that I know our people want”.

Cabinet minister Brandon Lewis said during a round of interviews on Sunday he thinks the PM is serious in his aspirations, arguing his desire to look “long-term” when it comes to his leadership “has got to be a good thing”.

The Northern Ireland Secretary told Sky News he sees in Mr Johnson “drive and enthusiasm for what we want to achieve for our country”, and that kind of “zest” is to be celebrated.

He told LBC there is no point in the PM “pretending he’s somebody else” after Mr Johnson insisted he will not undergo a “psychological transformation” despite pressure piling on his leadership.

Brandon LewisNorthern Ireland Secretary Brandon Lewis said the PM looking long-term is a positive thing (Jeff Overs/PA)

In an interview with ITV at the G7 summit, the Prime Minister said the Government will continue to do “remarkable” things.

Put to him that he no longer has voters’ trust, and asked if this is a source of personal shame, he said: “No, because I think that actually when you look at what this Government has done, it is quite exceptional.”

He added: “I understand that people are going to want to criticise me, attack me for all sorts of reasons, some of them good, some of them less good.

“I think that actually when you look at what this Government has done, it is pretty remarkable. We’re going to continue to do that.”

Asked for his biggest regret of his tenure so far, he said: “I’m going to leave that to further reflection, I haven’t had time to think about that.”

The Prime Minister has urged Tory MPs plotting to oust him not to focus on the issues he has “stuffed up”, after his authority was further diminished by a Cabinet resignation.

Oliver Dowden stood down as Tory Party co-chairman in the wake of the by-election defeats, saying he and Conservative supporters are “distressed and disappointed by recent events” and telling Mr Johnson that “someone must take responsibility”.

POLITICS Byelections(PA Graphics)

But the PM set his sights on being in office in the “mid 2030s”, in a run that would see him outlast Margaret Thatcher’s reign.

Asked by journalists at the British high commissioner’s residence in Kigali if he would lead his party into the next election, he said: “Will I win? Yes.”

In buoyant mood, the Prime Minister added: “At the moment I’m actively thinking about the third term and what could happen then, but I will review that when I get to it.”

Labour, meanwhile, challenged the Tories to call an early election, with leader Sir Keir Starmer telling Mr Johnson: “Bring it on.”

There are suggestions of a challenge to change the rules of the 1922 Committee of Conservative MPs in order to allow another vote of confidence in Mr Johnson within the next year.

Mr Lewis dismissed the idea, telling Times Radio “we shouldn’t even really be talking about it”.

The Prime Minister suggested Vladimir Putin would have not invaded Ukraine if he had a committee of Tory backbenchers on his case.

In an interview with CNN in Germany, he was asked about his message to Conservative MPs, who say “he is a drag on his ticket”.

He said: “I think the great thing about democracy is that leaders are under scrutiny and that I do have… people on my case, I have got people making arguments.

“Both China and Russia, I think make big mistakes because they don’t have those democratic checks and balances.

“Do you really think that Vladimir Putin would have launched an invasion of another sovereign country if he’d had people to listen to properly… if he’d had a committee of backbenchers, the 1922 Committee, on his case?”

Asked in Rwanda if he believed questions over his leadership were settled, Mr Johnson replied: “Yes.”

Tiverton and Honiton by-electionLiberal Democrat leader Sir Ed Davey celebrates the by-election victory in Tiverton (Andrew Matthews/PA)

But the attacks kept on coming from his own backbenches on Saturday night, with Damian Green, who chairs the One Nation caucus of Tory MPs, warning the Government “needs to alter both its style and content” and calling on Cabinet members with leadership hopes to show their stripes.

In the by-election in the Devon constituency of Tiverton and Honiton, a dramatic swing of almost 30% from the Conservatives saw their 24,000 majority overturned by the Liberal Democrats.

In West Yorkshire, Labour seized back Wakefield with a majority of 4,925 on a swing of 12.7% from the Tories.