As U.K. Prime Minister Theresa May presses on to take Britain out of the European Union, one leading economist raised questions about the viability of the single political and economic bloc.

"You have to wonder about the future of the European Union itself ... this is an imperfect union and the survivability of it is, I think, a serious question," Stephen Roach, a senior fellow at Yale University, told CNBC's Sri Jegarajah on Friday.

Even before Brexit came about, the EU faced multiple challenges over the last decade, said Roach, who's a former chairman of Morgan Stanley Asia. Those challenges include a sovereign debt crisis in Greece and a standoff with Italian leaders over the country's spending plans.

And with the U.K. — one of the largest European economies — planning to leave the bloc, it remains to be seen whether the EU has the ability to withstand more pressure coming from member states while still reeling from the shocks of the global financial crisis, said Roach.

The EU agreed to postpone the U.K.'s exit from the bloc to May 22 if May can convince the British parliament to accept the existing deal. If May fails to convince parliament, Britain would face a disorderly exit from the EU on April 12.