An MP who was found guilty of lying to avoid a speeding ticket will be sentenced at the Old Bailey on Tuesday, a decision that could spark a closely contested byelection within weeks.

If Fiona Onasanya, elected on a Labour ticket to represent Peterborough, receives a custodial sentence of more than a year, she will automatically be forced to step down.

With expectations high that she will receive a substantial term, Labour and the Conservative party are campaigning hard in the Cambridgeshire city that she won by just 607 votes in 2017.

Both parties are aware that a snap byelection could be the first opportunity for voters to use the ballot box to give their verdict on Brexit negotiations since Theresa May came back from Brussels with her deal.

Disowned by Labour and Jeremy Corbyn, Onasanya has said she will appeal against her conviction and fight to remain as an MP, but this will not prevent her from being sentenced on Tuesday.

If she is sentenced to a year or less in prison, even if it is a suspended sentence, a recall petition would be triggered allowing constituents to hold a byelection to vote in a replacement.

But any recall process would not begin until the appeals process had ended. This could take months, during which time Onasanya will continue to receive her public salary of £77,379 a year.

Expectations among local activists are that the Conservatives would be the marginal favourites to win a byelection. During the EU referendum, Peterborough voted to leave the EU by 61%.

The city usually voted with the victorious political party in general elections until Onasanya beat the influential Tory Brexiter Stewart Jackson two years ago.

On Monday, the work and pensions secretary, Amber Rudd, went to the city, the first of a series of expected visits by ministers.

Tory parliamentary candidate Paul Bristow, a leave supporter and political consultant who grew up in the city, said the party had delivered 30,000 leaflets in the last two weeks but would not be seeking to exploit Onasanya’s case with the voters.

“The case around Fiona is a personal tragedy,” he said, adding that Brexit was expected to be a major campaign issue, as well as more local concerns such as potholes, schools and safer streets.

The Greens, the Liberal Democrats and even the SDP have already chosen their next candidates. Labour has not, as concerns deepen about how a political novice such as Onasanya was chosen to stand. Unite activist Lisa Forbes, a previous Labour candidate in the seat, is expected to put herself forward along with local councillor Shaz Nawaz.

Despite the party’s uncertainty over a candidate, Labour is canvassing across the borough in expectation of a byelection, and insists that Onasanya’s name is not being raised on the doorstep. Instead voters want to talk about austerity, the local problem of fly-tipping and the state of the NHS, party sources claim.

Emily Thornberry, the shadow foreign secretary, and Ian Lavery, Labour’s chairman, have joined dozens of activists delivering leaflets at weekends since the beginning of the month.

A spokesman for Peterborough Labour party said they would choose a candidate “as soon as possible” if Onasanya received a sentence of more than 12 months, which would automatically disqualify her from being an MP. If she received a lesser sentence, the party would campaign to oust her. “If there is a byelection, Labour will be ready to fight and win,” he said.

Onasanya was convicted at a retrial of colluding with her brother, Festus, after her car was clocked travelling at 41mph in a 30mph zone in the village of Thorney near Peterborough in July 2017. After being found guilty, she sent a message to other Labour MPs likening her predicament to that of biblical figures including Jesus and asking them to pray for her.

She has rarely been seen in the city since the verdict, but she did vote against the EU withdrawal bill and will not be able to vote on Tuesday night if she receives a prison sentence.

She is still writing a column in the Peterborough Telegraph – on Sunday, she wrote about tuition fees – but does not mention her conviction or her sentencing. Neither Onasanya nor her legal team have responded to requests for comment.

Immigration remains a major issue in Peterborough, where the population has increased from 157,000 to 200,000 since 2001.

One local politician privately acknowledged that a minority of voters were unhappy with the number of recent arrivals from countries including Poland, Portugal, Lithuania, Albania and Afghanistan. “Without extra resources, there’s too much strain on local services,” he said. The Conservative-led council has received a major funding cut from central government.