Labour's John McDonnell has blamed a campaign "dominated by Brexit" for a predicted poor performance by Labour in the election.
The shadow chancellor said his party had been hoping "other issues would cut through".
A BBC/ITV/Sky exit poll put Labour on 191 seats, which would be its worst result since 1935.
But others within the Labour Party blamed Jeremy Corbyn's leadership for its projected performance.
The first general election results were announced before midnight.
In one of the first seats to declare, the Conservatives took Blyth Valley in Northumberland. It had been a Labour seat since its creation in 1950.
The exit poll predicts the Tories will have a majority of 86 in Parliament. The final result is expected to be known by Friday lunchtime.
During the campaign, Labour promised to renegotiate Prime Minister Boris Johnson's Brexit deal with the EU, and then put it to a referendum vote alongside the option of remaining in the EU.
That position could come under criticism, with the exit poll suggesting Labour would lose a number of seats to the Conservatives in areas that voted to Leave during the 2016 referendum.
Speaking to the BBC, Mr McDonnell said if the exit poll is correct, it would be an "extremely disappointing" result for his party.
"I thought it would be closer. I think most people thought the polls were narrowing," he said.
He added that he did not think Mr Corbyn's leadership of the party was "the big issue" of the campaign, but rather the issue of Brexit had "dominated".
Asked if Mr Corbyn should resign if the prediction is borne out, he said "appropriate decisions" would be taken when the results were in.
But former Labour home secretary Alan Johnson said Mr Corbyn's unpopularity was a factor in Labour's forecast losses.
He told ITV News: "It's Corbyn. We knew that in Parliament."
"We knew he was incapable of leading, we knew he was worse than useless at all the qualities you need to lead a political party."
Ian Murray, Labour's candidate in Edinburgh South, tweeted: "Every door I knocked on, and my team and I spoke to 11,000 people, mentioned Corbyn."
"Not Brexit but Corbyn. I've been saying this for years. The outcome is that we've let the country down and we must change course and fast."