David Davis will hold Brexit talks in Brussels on Monday ahead of a crunch meeting of European Union leaders later this week.
The Brexit Secretary is meeting counterpart Michel Barnier amid warnings from senior Tories that the UK must be allowed to formally sign trade deals during any transition period.
Britain will become a "joke nation" if the EU stops it agreeing new trading terms outside the bloc as it withdraws, Jacob Rees-Mogg claimed.
Mr Davis is in Brussels to thrash out details of transition arrangements that would come into force when Britain leaves in March 2019.
The Brexit Secretary has said he can "live with" an implementation period of under two years if it helps to secure an early deal.
Prime Minister Theresa May will head to Brussels on Thursday for a meeting of the European Council where she is hoping a deal on the arrangements will be signed off.
In an article for The Daily Telegraph Mr Rees-Mogg, who chairs the Brexit supporting Conservative European Research Group, wrote: "Britain has to decide if it is a serious country or a joke nation.
"It would be humiliating for others to have cause to think thus of us.
"The United Kingdom will not accept being a subservient state.
"In the case of tariffs, once we have left the EU, it is non-negotiable that our trade minister should be able to respond to any threat of increased tariffs from other nations as suits our national interest, not the EU's.
"We must be able to negotiate and sign trade deals.
"Yet this does not depend just on our legal ability to do so.
"It depends on how other countries view us."