Alex Salmond's plans for an independent Scotland to join the EU within 18 months of a yes vote are "unachievable" and "not credible", a Commons committee has concluded.

MPs on the Labour-dominated Scottish affairs select committee said the first minister was making impossible promises to voters by claiming Scotland could retain the UK's hard-fought opt-outs on the euro and the Schengen borders treaty and its lucrative EU rebate, and still join the EU by 24 March 2016.

It was more likely an independent Scotland would find itself temporarily outside the EU under interim arrangements, than the EU's 28 member states allowing a newly independent state to keep all of the UK's special privileges, which no other new member was allowed, the committee said.

The UK's opt-outs that Scotland would lose include a large range of VAT discounts, including on children's clothes, books and food. The European commission has stated that a new member can have VAT lower than 15% for only one or two categories of goods, not the dozens enjoyed by the UK.

Meanwhile, the Scottish government also wants the power to discriminate against English, Welsh and Northern Irish students by charging them university tuition fees given free to Scottish and all other EU students after independence.

Ian Davidson, the committee's Labour chairman, said it accepted that an independent Scotland would be given EU membership eventually, but he added: "We believe that the Scottish government cannot meet its negotiating objectives within the timetable it has promised. The people should be told which concessions will be made and what the costs will be. Will the timetable for independence slip or will there be a period outside the EU? What is their plan B?"

The committee – which has no active SNP membership after its MP Eilidh Whiteford began a boycott following a row with Davidson – added that no European member state or commission official had supported Salmond's contention that Scotland could use a fast-track process under article 48 of the EU treaty.

Salmond's case was weakened further because he would need the UK government's agreement to use article 48 under the first minister's timetable for negotiating accession before Scotland became independent.

There was no prospect of the UK doing so if Scotland planned to discriminate against UK students on fees or fight to share UK opt-outs, the committee said. Fiona Hyslop, the Scottish government culture and external affairs secretary, said it was ironic that the report had been published the day after the UK's future in the EU had been threatened by Ukip's surge in the European elections.

"Despite their hostility to the idea of an independent Scotland, even these MPs have had to accept that an independent Scotland will be welcomed into the EU," she said. "The reality is that Scotland is an important part of the European Union and with the powers of independence Scotland will finally be able to speak for ourselves on issues such as farming, fisheries and opportunities to boost jobs to secure the right deal for Scottish industries."