Britain’s new immigration system will not necessarily be modelled on the Australian-style points-based system promised by leading Leave campaigners during the referendum, the immigration minister has told MPs.

James Brokenshire, a close confidante of Theresa May who has been tipped as her potential successor as home secretary, confirmed that work has started within the Home Office on detailing the options for Britain’s new immigration system that will curb the free flow of EU migration to the UK. “The prime minister has said that free movement cannot continue it its present form,” he said.

The immigration minister told the home affairs select committee on Tuesday that talks in the last three weeks with the Irish government had reaffirmed the desire of both the UK and Ireland to preserve their joint common travel area that has existed since 1922 when Britain leaves the European Union.

Brokenshire, who advised the MPs not to believe everything they read in the newspapers about his own immediate prospects, confirmed that the government will not unilaterally guarantee the future position post-Brexit of the estimated 2.9 million EU nationals currently lawfully resident in Britain until the position of UK citizens in EU countries is also secured.

He did however try to reassure EU citizens who have been long-term residents in Britain by conceding it was very difficult to remove from the country people who have lived here for at least five years. Under EU law, citizens of other EU countries have a permanent right to live in the UK if they have been in Britain for five years.

“Having established that right, I think, as a matter of law, it would be virtually impossible ... to then take that away from them,” said Brokenshire.

He disclosed that a Home Office “international immigration group” has already started work on mapping out the options for a post-Brexit immigration system that is feeding into the Cabinet Office’s central co-ordination Brexit unit under Olly Martins.

What is Australia's points-based immigration system? Read more

UK Visas and Immigration are also working on the operational impact of various possibilities although the Home Office along with the rest of Whitehall undertook no contingency planning before the referendum result.

“The Home Office work is to look at the various different options,” said Brokenshire. “It is not necessarily that the points-based system is the right way to do it. There are other arrangements that could be considered as well,” he said in reference to the Ukip immigration policy that was embraced by Michael Gove and Boris Johnson during the referendum campaign.

The Australian system sees about 15% of work visas issued to migrants on the basis of their skills and Brokenshire’s decision to distance May’s government from the central leave pledge shows that they are not going to be tied to specific pledges made by Brexiters.

He said he could not spell out the options any further in advance of the negotiations in the interests of getting the best possible deal for Britain.

The immigration minister, however, did hold out the prospect that free movement will continue between Britain and Ireland as it has done since 1922. He said both government had agreed their “joint desire to preserve the common travel area” even though the border between the Republic and Northern Ireland will now become an external EU frontier.

Brokenshire said that the fact this arrangement pre-dated Britain’s membership of the European Union and that it “would not impact on the security of the Schengen area” – the passport-free zone between the other 26 EU states meant ministers were hopeful it could be preserved. However, the European commission has yet to give any indication of its approach on the issue.