Asked at the State of London debate on Tuesday evening if the Labour Party nationally should be pushing to rejoin the single market, Mr Khan said: “I don’t speak for the national Labour Party.
“But I believe we should…spot on.
“The biggest piece of self-inflicted harm ever done to a country - leaving the European Union.”
Sir Keir Starmer’s party nationally are not advocating rejoining the single market.
Ministers are seeking to portray Labour and the Liberal Democrats as seeking to reverse Brexit.
But a Labour Party spokesman said: “Labour Party policy is clear. We need a strong collaborative relationship with EU partners but that does not involve membership of the customs union or the single market.”
Mr Khan’s comments came after an analysis of 211 polls by The Standard highlighted that a growing number of Britons now say Brexit was wrong.
It showed that an average of just under 49 per cent of adults believe it was a mistake, compared to just over 38 per cent who still say it was the right decision, with 13 per cent “don’t know”, according to 11 surveys carried out this year.
The average annual gap between those who believe it was “wrong” to vote to Leave compared to “right” has risen into double digits for the first time in 2022, to 10.6 percentage points.
This is almost double the 5.5 percentage point gap of last year, and higher than 6.4 percentage points in 2020 and just under seven points in 2019, according to the study of polls from 2016 which asked whether in hindsight people thought Britain was right or wrong to vote to leave the EU.
The latest figures also compare to the 52 to 48 Brexit referendum result in 2016 for Britain to quit the European Union.
Meanwhile, the Government appears to have adopted a policy of not telling the public about the economic impact of leaving the European bloc, with Brexit minister Jacob Rees-Mogg refusing to say if any such assessment have been carried out since the UK departed in January 2020.
A recent study by The Resolution Foundation, in collaboration with the London School of Economics, warned that Brexit will hit workers’ real wages by around £470-a-year, compared to what it would have been, and damage Britain’s competitiveness.
Another analysis, by the Centre for European Reform, estimated that the UK was being hit with a £31 billion blow to GDP from Brexit in the fourth quarter of 2021.