Meanwhile, she has so far failed to persuade Brussels to start talks on Britain's future relationship with the European Union, six months after setting a two-year clock ticking on Britain's exit from the bloc.
Brussels says it will not discuss its future relationship with Britain until London agrees to pay its outstanding dues, resolves the fate of EU citizens living in Britain and works out a plan for the future UK-EU land border in Ireland.
Speaking on the Today programme on BBC Radio 4, Mr Jenkin said Britain needed to stand tall and not make further concessions after Theresa May's Florence speech last month.
"Achieving that partnership will require leadership and flexibility, not just from us but from our friends, the 27 nations of the European Union", she will say, according to excerpts of her speech.
"So the ball is entirely in the United Kingdom court for the rest to happen".
"Because what we are seeking is not just the best possible deal for us - but also the best possible deal for our European friends too".More news: US President Donald Trump's threats 'could lead to WWIII' - Corker
"By approaching these negotiations in a constructive way - in a spirit of friendship and co-operation and with our sights firmly set on the future - I believe we can prove the doomsayers wrong", she will say.
Britain's ruling Conservatives barred two of their own lawmakers in the European Parliament after they broke the party whip and voted for a motion demanding London do more to meet EU demands in Brexit talks.
May's office says she will say Monday that "the ball is in their court".
"They're just stringing us along and there comes a point where you've got to dig in and say 'look, if you want to talk, we'll talk but otherwise we're going to get ready to leave".
Asked if May was confident of a breakthrough in the current round of talks, her spokesman said: "The (Florence) speech was meant to create momentum, we believe we are seeing that momentum, but lets see what happens in the next round of talks".
Aides to May have signalled that the prime minister has accepted that her October deadline will not be met despite a speech in Italy last month which attempted to reset the tone of the hard negotiations. I think we have seen that momentum.