South Ribble has been described as “nip and tuck” by one Labour supporter.
A surprise result could be on the cards as a result of tactical voting.
The seat is held by Conservative Seema Kennedy, who won it with a majority of 5,945 in the 2015 General Election.
As the count continues at South Ribble Tennis and Fitness Centre, Cuerden Way, Bamber Bridge, Labour are understood to be putting up a strong challenge.
One exit poll is predicting an 83 per cent chance of a Labour victory in South Ribble - one of 14 Labour gains being forecast.
Mrs Kennedy said: “I think everybody got a shock with the exit polls.
“We’re looking at the votes and Labour are making a very strong showing.
“There’s obviously been some tactical voting.”
She said her team had worked “very hard”, adding: “We’ll just have to wait and see what happens.
“I don’t want to speculate at this stage.”
Labour South Ribble Borough and Lancashire county councillor Matthew Tomlinson said: “It’s looking nip and tuck, very tight.
“Clearly the national campaign Jeremy Corbyn has run has generated a lot of interest, particularly with people who haven’t previously voted.
“It’s been a pleasure going round knocking on doors, excited by what people are offering.”
He added: “There’s clearly been evidence of tactical voting, whereby people who normally vote for other parties are going to vote Labour, just to get rid of the Conservatives and this local evidence people nationally are calling a progressive alliance.
“If we lost by 2,000 this would still stand because Seema had a majority of 6,000 before.”
Labour candidate Julie Gibson said: “It would be fantastic, I’ll make no bones about it.
“I came in thinking we’ll give it our best shot.
“I’m really pleased, we’ve handled a positive campaign. I found a receptive response to what we are saying.
“I’m not taking anything for granted. I’ve really enjoyed it.”
The result is expected at about 5am.
The candidates are:
Seema Kennedy: Conservative.
Andrew Wight: Green Party.
Julie Gibson: Labour.
John Wright: Liberal Democrat.
Mark Jarnell: NHA National Health Action Party.
Mark Smith: UKIP.
The South Ribble constituency had been represented since 2010 by Conservative MP Lorraine Fullbrook.
Before that, Labour MP David Borrow held the seat for the 13 years from 1997 to 2010.
Blackburn-born Mrs Kennedy grew up in Iran and the Ribble Valley.
She worked as a solicitor and in her family’s property business in Preston, before being elected as South Ribble’s MP.
Since her election she was made vice-chairman of the Conservative Middle East Council and is Co-Chair of the Jo Cox Loneliness Commission.