The former head of the UK's Vote Leave campaign has branded David Davis "thick as mince", as he warned that a provision in the Brexit Secretary's so-called Great Repeal Bill would allow British ministers to cave in to EU demands at the last minute.
The Twitter outburst by Vote Leave campaign director Dominic Cummings reflects continuing rancour between different wings of the Brexit movement which saw intense infighting at the time of last year's EU referendum in Britain.
It comes after Mr Cummings admitted there was a chance that leaving the EU would turn out to be an "error" and described British Government members who thought the UK should leave the European atomic energy community Euratom as "morons".
In a stream of messages on Twitter, Mr Cummings warned that a little-noticed clause of the European Union (Withdrawal) Bill tabled by the British Government last week could allow ministers to alter their Brexit position at the last minute to secure a lengthy transition period, which he suggested was favoured by Mr Davis, Chancellor Philip Hammond and Cabinet Secretary Jeremy Heywood.
He said the Brexit Secretary was "manufactured exactly to specification as the perfect stooge for Heywood: thick as mince, lazy as a toad and vain as Narcissus".
Mr Cummings, a former special adviser to Michael Gove in the UK's Department for Education, worked behind the scenes on the Vote Leave campaign which was fronted by figures including Mr Gove and Boris Johnson, while fellow Brexiteer Mr Davis appeared on platforms for the Grassroots Out organisation, which was aligned with the rival Leave.EU organisation funded by Ukip donor Arron Banks.
The Vote Leave director has now said that Mr Davis "spent the campaign boozing with (Nigel) Farage, predicting defeat and briefing (against) Vote Leave" and was involved in the "single crappest TV news for Leave of entire campaign", when Grassroots Out unveiled George Galloway as a supporter.
Mr Cummings insisted his latest comments were not prompted by any falling out between Mr Gove and Mr Davis, insisting he had not spoken to the Environment Secretary about his Cabinet colleague since before last year's referendum.
He said the implications of Clause 9 of the European Union (Withdrawal) Bill may not be understood by MPs or even by British Prime Minister Theresa May.
The Clause gives ministers the power to use secondary legislation "to make legislative changes which they consider appropriate for the purposes of implementing the withdrawal agreement", including by amending the Bill itself.
Mr Cummings said this would embolden EU negotiators to make greater demands on the UK because they would know ministers were not bound by the text of the bill and "can be arm twisted up to last second".
The Government would be "empowered to ditch almost anything they promise up to last minute when arms twist", making it much easier to implement a protracted transition period to full withdrawal from the EU, he warned.