Theresa May has confirmed that the Conservative pledge to cut net migration to the tens of thousands will be in her party’s manifesto, despite having missed the target after making the same promise in 2010 and 2015.



“We do want to bring net migration down to sustainable levels. We believe that is the tens of thousands,” the prime minister told journalists at an event in the constituency of Harrow West to launch the campaigns of Conservative candidates in London and the south-east.



The comments triggered an immediate response from a liberal Conservative pressure group that claimed it was unlikely the target would ever be reached.

“Keeping the net migration target is a mistake. Controlling migration should not be centred on an arbitrary, indiscriminate and unrealistic figure. The prime minister should introduce realistic, effective and popular ways of controlling migration,” said Ryan Shorthouse, director of Bright Blue, which has the support of 140 Tory parliamentarians.



Senior party sources have told the Guardian that sticking to the policy was largely about perception so that voters see the government working towards the goal even if it is internally thought to be difficult to achieve.



They said the prime minister would also want to keep student numbers in the overall net migration figure, despite cabinet ministers such as Boris Johnson thinking they should come out, because taking them out would look like trying to “rig the rules”.

May, who oversaw immigration policy during more than six years at the Home Office, claimed that Brexit would help her achieve the aim. “When we leave the European Union we will have the opportunity to make sure we have control of our borders – leaving the EU means we won’t have free movement as it has been in the past,” she added.

She made the comments at an event centre on an industrial site in Harrow to scores of south-east and London candidates, including cabinet ministers Damian Green and David Gauke, the former justice secretary Michael Gove and former MPs Nick de Bois and Mary Macleod, who hope to win back their old seats. The group was joined by journalists and surrounded by banners that read “Theresa May’s team”, with the word “Conservatives” in much smaller print.



The prime minister also suggested that the victory of Emmanuel Macron in France would boost the EU27 in Brexit talks.



“Yesterday a new French president was elected – he was elected with a strong mandate which he can take into a strong position in negotiations. In the UK we have to make sure we’ve got an equally strong mandate. And every vote for me and my team will strengthen my hand in those negotiations,” she said, triggering a huge round of applause from the gathered candidates.



In a personal attack on the Labour leader, May added: “And the alternative is to risk making Jeremy Corbyn prime minister, and try to picture him sitting at that negotiating table with the collective might of the European commission and 27 other European countries against him.



“A vote for any other party is a vote to be a step closer to Jeremy Corbyn sitting at that Brexit negotiating table – we must not let that happen.”



She was also asked about Macron’s previous comments about ripping up the Le Touquet agreement, a bilateral treaty that allows British border officials to be stationed in France, but has been controversial because of migrant camps in Calais.



May suggested the issue would be high on the agenda in June. “It works for the benefit of the UK and France, and obviously the government that is elected on 8 June will be sitting down and talking to Monsieur Macron and others about how that system has worked both for the benefit of France and benefit of UK,” she said.