Downing Street has refused to rule out the possibility of the UK continuing to pay budget contributions to the EU after Britain’s departure from the bloc, as analysis suggested the country could face a €20bn (£18bn) “Brexit divorce” bill in shared payment liabilities.

A No 10 spokeswoman said Britain’s payment liabilities were part of “a whole range of issues and elements to our relationship with the European Union that will need to be addressed as we leave the EU”.

She said the issues of contributions to the budget were just one element of the negotiations. “These issues affect both sides, the UK and the EU, but we won’t get into a running commentary on the details,” she said.

Asked whether the prime minister would rule out any contributions to the budget once the UK leaves the EU, the spokeswoman said: “I think that gets into speculation territory. We are focused on preparing for the negotiations, and making sure we approach them in the right way to secure the right deal for Britain.”

Analysis by the Financial Times (paywall) found more than €300bn of shared payment liabilities would need to be settled as part of the UK’s exit negotiation, which include pension pledges and infrastructure contracts.

The €20bn “upper estimate” in the FT’s analysis would cover the UK’s share of continuing multiyear liabilities, including €241bn of unpaid budget appropriations, pension liabilities of €63.8bn and other commitments totalling about €32bn. The EU commission declined to comment on the €20bn figure, and Downing Street refused to be drawn on whether that figure was one it recognised as a fair estimate.

The projected costs, according to Labour MEP Clare Moody,“highlights that negotiations will not only be on our terms, – 27 other EU states also have their own agendas”.

Moody, who is a member of the EU parliament’s budget committee, said: “The projected cost is also in euros – if the pound continues to fall in response to Theresa May’s gung-ho negotiation strategy the bill for the UK will only increase.”

“We have already asked the government to clarify if they accept that any transitional arrangements would require the UK to continue making a sizable contribution to the EU budget for an indefinite period post-Brexit. We are yet to receive a reply.

“The final bill will have to be negotiated, and we have little faith that Theresa May alone will be able to negotiate a good one. Westminster must be involved in the negotiation process.”

Labour MP Pat McFadden, of the Open Britain campaign, said: “Brexit was supposed to be all gain financially but as the potential price of it becomes clear, people will not and should not forget the promises made during the referendum campaign.”

On Thursday, Boris Johnson, the foreign secretary, said he expected Britain to get a better deal for its goods and services when it strikes a deal to trade into the single market.

Appearing before the foreign affairs select committee, he said Britain can remain a “lodestar and magnet” for talented migrants from around the globe even after the country introduces tougher immigration rules after Brexit. He also said it was quite possible to strike a deal within the two-year timeframe set out once Britain triggers article 50 in March next year.

“We are going to get a deal which is of huge value and possibly of greater value ... We are going to get the best possible deal for trade in goods and services,” he said. “There are many countries that sell very effectively into the single market and that’s certainly what we will do.”

Asked repeatedly about whether the UK wanted to be a member of the EU single market, or simply have access to the market after Brexit, Johnson said the single market was an increasingly useless term and it was not the same as trying to seek membership of a club such as the Groucho in London.

That reference to trading into the single market suggests he does not envisage the UK remaining a member, even in a transitional phase.

Johnson said the remaining 27 EU states had a huge interest in agreeing a deal that would allow Britain to continue to trade its goods and services, adding that it was wrong to suggest that trade links were dependent on allowing free movement of people. Any attempt to punish the UK financial services would not make economic sense for Europe, he said.

“The idea that the Brownian movement of individuals, of citizens across the surface of Europe is somehow there on tablets of stone in Brussels is a complete nonsense,” Johnson said. “We are taking back control of our borders as we said we would, and that’s what we will do.

“It doesn’t meant that we are going to be hostile to people of talent who want to live and work here. I think it is extremely important that we continue to send out a signal of openness and welcome to the many brilliant people who help to drive the London economy and the UK economy.”

Brexit means “restoring our democracy and control of our borders and our lives and a fair bit of cash”, said the foreign secretary. “But Brexit is not any sort of mandate for this country to turn in on itself and haul up the drawbridge or to detach itself from the international community.”

Johnson said he thought Britain had done “the right thing” in voting to leave the EU on 23 June. He told the committee: “I think those who prophesied doom before the referendum have been proved wrong and I think they will continue to be proved wrong.”

He added there would be some “Sturm und Drang on the way ... Obviously it will take time before the full benefits of Brexit appear.”