BRUSSELS, Nov 30 (Reuters) - The European Union will launch contingency measures on Wednesday or Thursday if it has been unable to reach agreement by then with Britain on a trade deal, a senior EU diplomat said on Monday.

The diplomat said the move would be necessary because "it will be roughly three weeks left until the end of the transition period" that ends on Dec. 31.

"Companies and institutions like customs offices around the EU need to have clarity about what tariffs to impose and other measures if there is no deal and by the middle of the week we will have finally reached that point when such measures have to be spelled out," the diplomat said.

"We have been postponing that moment for weeks now, but this is the limit," the diplomat said. (Reporting by Jan Strupczewski Editing by Gareth Jones)