The findings come as the new Labour leader is set to mark his first 100 days since succeeding Jeremy Corbyn.
From July 9 to July 10, Opinium interviewed 2,002 UK adults on their opinions of their two opposition party leaders. According to the poll, 52 per cent of voters said they could imagine Sir Keir inside Number 10.
However when it comes to economic competence and whether Labour has the ability to run the economy, 42 per cent said they favoured Mr Johnson and Chancellor Rishi Sunak over Sir Keir and Shadow Chancellor of the Exchequer Anneliese Jane Dodds.
Laura Parker, the former national coordinator of Momentum, told the Guardian that more members of the Labour party were willing to work with Sir Keir.
She said: “We are going to have to build a broad coalition to beat this lot [the Tories] and after four election defeats the party left, centre and right have understood that."
According to the Opinium poll, Sir Keir is now strongly favoured by voters over Johnson when people are asked about the two leaders’ qualities – such as their competence, their ability to take decisions and get things done, and their ability to represent this country abroad.
The scores, which were generated by looking at the percentage of those who believe the leaders are not competent and subtracting from percentages who say they are, Sir Keir scored a net positive result of +28 per cent against -4 per cent for Mr Johnson.
The Prime Minister scored a negative result of -2 when it came to the question of whether he was a strong leader, while Sir Keir scored +21.
The Labour leader also performed better than Mr Johnson when it came to who was at better at negotiating with the EU as he scored +4, double the score of Mr Johnson's +2.
It comes after a later Opinium poll in June found that 37 per cent of voters thought Sir Keir would be better at leading the country, compared to 35 per cent for the current PM.
The latest figures released by YouGov revealed voting intention polls were consistently showing a large drop in the Tory leads.