The construction union offered to stop demonstrations in December at the Euston site if main contractor the Costain/Skanska Joint Venture (CSJV) handed out £25 turkey vouchers to workers.
But Unite chiefs said their offer was ‘given the bird’ and protests resumed on Tuesday morning.
The union claims officials are not being allowed into the site canteen and welfare areas during break times to discuss working conditions with operatives.
The job has already been hit by a pay row over overtime rates and has become a focus of Unite action in construction.
Unite national officer for construction Jerry Swain said: “We suggested a ‘turkey truce’, which would have acted as a goodwill gesture and created a less hostile environment in which to negotiate and resolve the real and growing problems on site.
“Yet CSJV is so arrogant it did not even acknowledge this peace offering and instead gave it ‘the bird’.
“As a result protests, which could cause considerable disruption, are continuing and will do so into 2020.”