5 October 2017
In his new book, Mr Clegg said it was "a time of national emergency" and he told people worried about Brexit to "make your voice heard".
He also said joining the Conservatives was "another route to make your views felt".
Mr Clegg, the former deputy PM, lost his seat in June's general election.
He has remained an outspoken critic of the UK's departure from the EU, and his new book is called "How to stop Brexit".
In it, he says it "may seem odd" for a former Lib Dem leader to be advocating joining Labour, adding that he himself would not be doing so.
"But if you are someone who has never joined a party, or perhaps has been inclined to join Labour but has never got round to it, or if you are simply someone who recognises that the importance of Brexit is far greater than individual," he writes.
"At a time of national emergency, and for as long as Parliament is dominated by Labour and Conservative MPs, it is undoubtedly true that what happens within the two larger establishment parties is of the greatest importance.
"So if you can't stomach joining the Labour Party, if you are ideologically inclined in a Conservative direction in any event and if you also believe that Brexit is the issue of our times, then joining the Conservatives is another route to make your views felt."
