Timothy, who served jointly with Fiona Hill until they both resigned in the wake of the disastrous election result, said May's intervention in Florence tomorrow would play a critical part in getting negotiations back on track, but stressed that “reassurance alone will not unblock the talks”.
She must show the UK’s “willingness” to pay an exit bill - although not an "exact solution" - and only once a sensible amount and the future relationship with the bloc has been agreed, he wrote in the Telegraph.
But she could be thrown off course in her efforts if the chancellor and foreign secretary don't “stop their games”, Timothy said.
"Boris Johnson and Philip Hammond… must understand that the surest route to a bad deal, or no deal at all, is to go on behaving as they are. They must stop their games now, because the stakes for Britain are too high,” he wrote.
The chancellor's refusal to embrace the "positives of Brexit" would lead to a bad deal with the EU, Timothy claimed.
"The Treasury's reluctance to mention the positives of leaving the EU is why the government has not talked positively often enough about the opportunities of Brexit," he added.