Mr Sunak appears to have lost significant ground as 60 per cent of the party members polled between 29 July and 2 August say they intend to vote for the foreign secretary.
This is up from 49 per cent since the period of 20 to 21 July.
Support for the former chancellor has dropped from 31 per cent to 26 per cent according to YouGov, while the rest of the 1,043 Conservative party members polled say they are undecided or will not vote.
It comes as voting in the Conservative Party leadership race will be delayed by more than a week after GCHQ warned that hackers could meddle with members’ ballots.
The National Cyber Security Centre – the listening post of GCHQ and one of the government’s three intelligence organisations alongside MI5 and MI6 – said there are vulnerabilities in the voting system, The Telegraph reported.