The leader of the CBI also called on UK politicians to rein in their public disagreements about Brexit.
Director General Carolyn Fairbairn wrote to members at the end of a “rollercoaster” year.
She said, citing slower growth, delayed investment and higher inflation: “Firms have responded with resilience but there’s no doubt that the impact of uncertainty has taken its toll.
“Thankfully the year has ended with some good Brexit news.
“We all breathed a sigh of relief when the EU Council in mid-December confirmed that trade talks can now begin and a status quo transition has been agreed in principle.
“Even harder work lies ahead. To keep jobs and investment in the UK, binding Brexit transition terms by the end of the first quarter need to be accompanied by progress on a framework for a final deal that delivers barrier free trade with the EU.
“From our politicians we need unity, clarity and certainty, not a different opinion every day.
“Politics will need to work on business timescales if we are to get the right result for the country.”
Ms Fairbairn also called on progress to adapt Britain’s economy to compete in a “transforming” world.
She warned: “The world won’t wait for Britain.
“The top priority must be the urgent transformation of our skills base.
“The clock may be ticking on Brexit but it is ticking just at fast in our schools, colleges, universities and workplaces.
“The UK is facing a skills emergency.”
She pledged the CBI to campaign for reformed careers advice and enhanced work experience in schools, effective technical education and better skills training for adults.