Labour is targeting voters in key marginal seats with Facebook adverts that warn that switching to the Liberal Democrats will end up causing Brexit.

Jeremy Corbyn’s party is using the ads to encourage tactical voting against the Tories, amid a row in Westminster about the best way to maximise opposition to Boris Johnson.

The move comes after criticism of tactical voting websites for recommending voters back the Lib Dems in tight Labour-Tory races. Labour supporters on social media claimed that the advice might hand the seats to Tories by splitting the Remain vote – making Brexit more likely.

One advert pushed out to the Facebook feeds of around a quarter of a million people says: “A vote for the Lib Dems gets you Brexit. Labour will give the people the Final Say. It’s time for real change.”

The advert was disproportionately targeted at millennial voters aged 25 to 34, but was also shown to large numbers of 35- to 44-year-olds and some 18- to 24-year-olds. It ran in English, Scottish and Welsh constituencies.

Top 10 marginal seats to watch at the general election Show all 10 1 /10 Top 10 marginal seats to watch at the general election Top 10 marginal seats to watch at the general election North East Fife Currently held by the SNP with a majority of 2 Getty Top 10 marginal seats to watch at the general election Kensington Currently held by Labour with a majority of 20 Getty Top 10 marginal seats to watch at the general election Perth and North Perthshire Currently held by the SNP with a majority of 21 Getty Top 10 marginal seats to watch at the general election Dudley North Currently held by independent, formerly Labour, MP Ian Austin with a majority of 22 LivingInMediocrity Top 10 marginal seats to watch at the general election Newcastle-under-Lyme Currently held by Labour with a majority of 30 Derek Harper Top 10 marginal seats to watch at the general election Southampton Itchen Currently held by the Conservatives with a majority of 31 Rob Candish Top 10 marginal seats to watch at the general election Richmond Park Currently held by the Conservatives with a majority of 45 Robin Webster Top 10 marginal seats to watch at the general election Crewe and Nantwich Currently held by Labour with a majority of 48 Jaggery Top 10 marginal seats to watch at the general election Glasgow South West Currently held by the SNP with a majority of 60 Alec MacKinnon Top 10 marginal seats to watch at the general election Glasgow East Currently held by the SNP with a majority of 75 Christine Johnstone

Other similar adverts targeting potential Lib Dem votes warned: “The Lib Dems can’t win here. Vote Labour for the final say on Brexit. It’s time for real change” and “Only Labour can stop Boris Johnson’s disastrous Brexit deal.”

The Lib Dems have made Brexit the focus of their election campaign, having done well off the back of the issue in the European parliament elections in May.

Facebook is turning into a key battleground between the Lib Dems and Labour, with the parties both spending hundreds of thousands of pounds on ads with the social network over the last year.

The Lib Dems have outspent Labour since October 2018 on the platform, ploughing £394,943 into targeted adverts since that date. Labour has meanwhile spent £280,201, according to Facebook’s public figures.

The Labour advert was shown to hundreds of thousands of voters (Facebook)

Lib Dem adverts aimed at Jeremy Corbyn and his party bear a simple message, usually featuring a picture of the Labour leader with a claim such as “Jeremy Corbyn is a Brexiteer at heart” or simply “Jeremy Corbyn is pro Brexit”.

The party also claimed in adverts pushed out last week that the “Liberal Democrat are surging” and “rising in the polls” – though in fact the party's support has actually fallen away since the start of the election campaign compared to previous months.

Labour’s policy on Brexit is to hold a second referendum between Remain and a new, softer Brexit deal within the first six months of taking office. The Lib Dems say they would revoke Article 50 if they won a majority government but would insist on a referendum in coalition negotiations. However, the party has also emphatically said it would not do a deal with Mr Corbyn under any circumstances.

The Lib Dems, Green Party, and Plaid Cymru on Thursday unveiled a plan not to stand against each other in 44 seats across England and Wales, dubbing the pact a “remain alliance”.