Labour is braced to lose the Norwich North by-election, the first contest prompted by the MPs' expenses row.
The Conservatives have won the Norwich North by-election with a majority of more than 7,000.
Sources at the count suggest the Tories are poised to take the seat.
The winning candidate Chloe Smith, 27, will become the House of Commons' youngest MP when she takes her seat after the summer recess.
Tory leader David Cameron made his sixth visit to the constituency on Thursday as part of his party's efforts to overturn Labour's 5,459 majority.
It was the first Westminster election since the MPs' expenses furore.
The result is due at about mid-day. Turnout was 45.8%, less than the 61% who voted at the general election but about average for recent by-elections.
The former MP, Labour's Ian Gibson, quit when the party barred him from standing at the next general election over his expenses claims.
The Labour vote is thought to have been split by anger at the way former MP Ian Gibson was effectively de-selected by the party.
'Very positive'
The Greens, the main opposition party on the council, were hoping to benefit from anger at the main parties and Labour fears the party could slip to third place, behind the Greens or Liberal Democrats.
At the count in Norwich BBC correspondent Carole Walker said it looked as though Labour would come second, after earlier speculation it could be pushed into third, but the Labour vote had slumped.
Theresa May, who ran the Conservatives' campaign in the constituency, refused to predict a Tory win.
But she said: "I know we had a very positive response on the doorstep and I was very pleased with that."
She said the expenses row had led to a "plague on all the main parties' heads" and it had an impact on voting.
"We did some Labour voters coming to us, but some Labour voters stayed at home and I'm sure went to other parties as well," she said.
On Thursday night former cabinet minister Geoff Hoon said it would be "difficult" for Labour to hold the seat.
'Victim of moment'
On BBC One's Question Time, he said the contest was taking place at a difficult time for the government in the middle of a recession and with public feelings still running high over the expenses scandal.
"This by-election has been a perfect storm for a governing party," he said, acknowledging many constituents were unhappy about the way Dr Gibson had been treated.
Dr Gibson had been barred from standing again by the Labour National Executive Committee over his expenses claims and chose to resign immediately.
His local party continued to back him and Mr Hoon said Dr Gibson had been a "very good MP" as well as a "very talented man".
"I well understand why people who worked with him, supported him and voted for him were concerned about what happened," he added.
He said the circumstances meant it would be "pretty difficult" for Labour to win.
Senior Labour MP Tony Wright told BBC Two's Newsnight he thought Dr Gibson had been badly treated.
"There were people in the House of Commons who did far worse things than he did," he said.
"He was a victim of a moment when all the parties, and all the party leaders, were falling over each other to show how tough they were being."
BBC political correspondent Norman Smith said the by-election was being seen as a "virtual lost cause" for Labour, but there was little mood in the party for another debate about Gordon Brown's leadership.
Labour's campaign faced further problems when its candidate Chris Ostrowski was hospitalised with suspected swine flu on Tuesday.
Should Mr Ostrowski, 28, or his Conservative rival Chloe Smith, 27, win the seat they would be the youngest MP in the House of Commons. The current "baby of the House" is the 29-year-old Lib Dem MP Jo Swinson.
