Zac Goldsmith’s lead in the Richmond Park byelection is narrowing with a week to go, a senior Liberal Democrat source has claimed, as candidate Sarah Olney made her final pitch to voters.

Data collected by the Lib Dems on the doorstep suggest Mr Goldsmith will take about 50 per cent of the vote, with Ms Olney trailing by around 8 per cent.

It would mean Mr Goldsmith’s majority would be cut from more than 23,000 to about 5,000, based on the turnout in the 2015 general election.

A senior Lib Dem source said: “This is a huge increase in our vote. With the Labour vote crumbling and no Green candidate standing, this is a close two horse race.”

The Liberal Democrats cut the Conservative majority in Witney, in Oxfordshire, from more than 25,000 to less than 6,000 in October.

Most of the Liberal Democrat swing came in the final week, the party source claimed.

He said: “If we’re in this position now, with a week to go, it’s good. Without making any macho predictions, this is very encouraging.

“We’re on the final bend. Zac is ahead but we’re on his shoulder. It then becomes a sprint down the home stretch.”

Mr Goldsmith criticised the “bombardment” campaigning approach of his opponent, from which the data were collected, saying it was “annoying people”.

He claimed one constituent told him he received 24 separate pieces of campaign literature from the Liberal Democrats.

Mr Goldsmith said: "I think the Liberal Democrats have misjudged this election - but it's ultimately up to the voters to decide."

Having only joined the Liberal Democrat party in May 2015, North Kingston resident Ms Olney has pitched herself as a Westminster outsider who is more representative of the constituency than Mr Goldsmith.

She said: “I am a local resident, my children are in local schools, and I understand local issues. I juggle raising children and having a full-time job. I have a lot in common with local people.

“I am more like the average Richmond Park constituent than Zac; I have more experience of similar things that residents face.”

Mr Goldsmith said the fact he received the largest increased majority of any sitting MP in the last general election is proof of his appeal to constituents.

This month the Liberal Democrats called for a second EU referendum before Article 50 is triggered, beginning the process of leaving the union.

Ms Olney, who backed the Remain campaign, said the High Court ruling on November 3 that Parliament would vote on whether to trigger Article 50 was a “ray of hope”.

The Liberal Democrats published the results of a survey of 3,500 constituents on Thursday, which showed 83 per cent believed Brexit would have a negative impact on local jobs, and 87 per cent thought remaining in the single market should be a priority.