BERLIN, April 14 (Reuters) - Germany’s economic institutes will cut their joint 2021 growth forecast for Europe’s largest economy to 3.7% from 4.7% previously due to a longer than expected COVID-19 lockdown, two people familiar with the decision told Reuters on Wednesday. The institutes are expected to release their joint growth forecast on Thursday. The German government said in January it expected gross domestic product to grow by 3% this year, following a drop of 4.9% in the previous year caused by the coronavirus. (Reporting by Holger Hansen, Christian Kraemer and Rene Wagner; Writing by Michael Nienaber; Editing by Maria Sheahan)