John McDonnell said that the slowdown in growth in the last quarter from the previous quarter showed a "slow erosion" of the British economy.
The shadow chancellor said: "Manufacturing fell back in this quarter and that's an indication of the slowdown that's happened in our economy overall.
"We've always said that we never expected an immediate crisis. What we were worried about was a slow erosion of our economy if we don't get certainty and security for the future."
In a speech on Thursday, Mr McDonnell said that the Government had to be "clear about its objectives and its approach to prevent uncertainty".
More top news
Theresa May scraped through Commons votes on customs with support from Labour MPs.
Another minister resigned as the Government narrowly won a vote on its customs bill after Theresa May bowed to pressure from Tory Brexiters.
Theresa May had to rely on the support of Labour MPs on a controversial proposal.