Keir Starmer has said that Boris Johnson has 'failed' by defending Dominic Cummings over a 260-mile trip during lockdown.
The prime minister has rejected calls to sack his closest adviser.
He said the public would “understand” Mr Cummings had travelled to Durham to secure childcare for his young son.
But he skirted questions over allegations Mr Cummings took a 30-mile day trip to Barnard Castle while he was there and while the country was being urged to stay at home.
The new Labour leader said Mr Johnson’s comments were a “insult to sacrifices made by the British people”.
Sir Keir said: "This was a test of the Prime Minister and he has failed it.
"It is an insult to sacrifices made by the British people that Boris Johnson has chosen to take no action against Dominic Cummings.
"The public will be forgiven for thinking there is one rule for the Prime Minister's closest adviser and another for the British people.
"The Prime Minister's actions have undermined confidence in his own public health message at this crucial time.
"Millions were watching for answers and they got nothing. That's why the Cabinet Secretary must now launch an urgent inquiry."