The diplomat suggested his job had been made harder by the previous government’s EU negotiations. It comes as the UK prepares to go into its next round of talks with the EU next Tuesday.
Both sides are still trying to overcome policy hurdles that have so far made a post-Brexit trade deal elusive.
Mr Frost told the Mail on Sunday: “We came in after a government and negotiating team that had blinked and had its bluff called at critical moments, and the EU had learned not to take our word seriously.”
He said “a lot” of his current job was convincing the EU that “they should take our position seriously.”
The Brexit expert also had pointed words for the EU, adding: “I think they spend too much time trying to guess what our intentions are and not enough time listening to our words.”
On Friday, he also appeared to pre-emptively blame the EU for any further impasse in next week’s negotiations.
He said in a tweet: “We will negotiate constructively but the EU’s stance may, realistically, limit the progress we can make next week.”
The likelihood of a UK-EU post-Brexit trade deal has been increasingly uncertain in recent weeks as both sides failed to agree on key areas such as fishing rights and business competition rules.
The UK is set to leave the single market and customs union on December 31, but both sides would need to agree on a deal by the end of October at the latest in order for it to go through parliament.
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On Friday, the Telegraph claimed the EU’s negotiator Michel Barnier would be “sidelined” so heads of EU member states could intervene in the talks as the deadline approaches.
A plan for how to do this was to be outlined in a speech by European Commission president Ursula von der Leyen on September 16, the paper reported.
However, European Council president Charles Michel is now said to have ruled this out, with the EU instead focusing on other issues such as its coronavirus recovery.
In any case, next week’s negotiations are due to run from 1:30pm on Tuesday until Thursday afternoon.
The government has published the agenda for the talks online, which reveal the topics to be discussed.
Each topic, including energy, business competition and the trade of goods, is given roughly eight hours of discussion over the entire week’s negotiation period.