Ministers are looking at preparations for a "no deal" Brexit on a weekly basis, the head of the Civil Service has said.
Cabinet Secretary Sir Jeremy Heywood said the Government was "completely on it" in terms of planning for leaving the European Union with no deal.
But he warned that quitting without striking an agreement on future trade relations would only be a success if other EU countries and businesses also prepare.
Prime Minister Theresa May is seeking a comprehensive free trade deal with the EU but has been at pains to say she is ready to walk away from negotiations, insisting "no deal is better than a bad deal".
Many Brexiteers believe Britain must show it is prepared to break off talks in order to strengthen its negotiating hand.
Appearing before the Public Administration and Constitutional Affairs Committee, Sir Jeremy said: "There are huge challenges with no deal, obviously we've done a vast amount of work on the no deal scenario and that work continues and we will do it to the best of our ability.
"But the most important thing to understand about the no deal scenario is that a lot of the work that will determine whether or not it is a sort of smooth transition to no deal is work that would have to be done by other countries.
"The Port of Calais for example - we can get our ducks in a row entirely in Dover but if that isn't the case in Calais then that causes some problems, causes some problems to the French as well of course.
"Similarly our private sector companies will have to do quite a lot of preparation and so on, so there are things external to government.
"On the things we can control ourselves, I would say that we were - I don't want to sound overly confident about this - but we have got a very, very good grip on the issues, we go through them on a very, very frequent basis... we've got a very good inter-ministerial group of ministers that now looks at this on a weekly basis, so we know what the issues are, we're working at them hard, I don't want to reveal all the vulnerabilities of our planning position, but we are completely on it, that's what I would say."