Why doesn't cron run python script from under virtual environment?

Question

Python

Here is a cron task:

*/20 * * * * source /home/user/venv/bin/activate && python /home/user/project/main.py && deactivate

Cron runs it regularly at the right time (checked in /var/log/syslog). But the script itself does not work.
It does not work both from under the root and from under the user.
Tried to run this command manually. Works fine.
Another task that does not use virtualenv works fine.

Answer 1

Instead of
call immediately python
/home/user/venv/bin/python