Downing Street does not accept the proposal for the UK to pay up to £50bn in a divorce settlement with the EU, Theresa May’s spokesman has said.

The £50bn bill has been widely reported as under discussion by senior EU diplomats at a European council meeting in Brussels on Thursday. It would represent Britain’s share of long-term liabilities such as pensions – one of the many issues that would need to be resolved during the article 50 talks.

But May’s spokesman rejected the £50bn figure, saying: “Negotiations have not begun and so that figure does not actually exist.” He added: “As was set out last night by my colleagues in Brussels, that is one of a range of issues that will have to be dealt with. The outcome of those negotiations will be something for the future.”

A hefty one-off divorce bill would eat into the funds that Vote Leave campaigners promised could be kept in Britain and spent on other priorities such as the NHS.

Separately, ministers including the Brexit secretary, David Davis, have conceded that Britain may end up making ongoing payments to the EU in exchange for access to the single market.

May left the summit to fly back to London late on Thursday night, cancelling a planned press conference after talks ran on into the evening. Her 27 EU counterparts remained there to discuss how they would manage the process of Brexit – but in the end devoted little more than 20 minutes to the subject.

No 10 also confirmed on Friday that May had sought an assurance from fellow leaders that the rights of UK citizens living in the EU and those of EU citizens living in the UK would be resolved early on in Brexit discussions. The prime minister told her EU counterparts that the topic should be a priority in negotiations, with Britain ready to guarantee the rights of EU citizens in the UK as soon as British citizens in other EU countries were protected in the same way.

She had sought a deal even earlier than the start of formal negotiations, likely to begin in March, but the EU has declined to start talks without article 50 being triggered.

“What happens in the negotiations is a matter for the negotiations. But we have made it very clear it is a matter we want to see resolved as soon as possible,” May’s deputy official spokesman said. He defended May’s decision to leave without giving a press conference, saying the Brussels council had overrun.

The shadow Brexit secretary, Keir Starmer, has called for May to make a unilateral commitment that she would protect the rights of EU citizens already living in the EU, as a goodwill gesture to kick off the article 50 talks in a positive spirit.

Helena Kennedy, the chair of the House of Lords subcommittee on the acquired rights created by EU membership, has urged EU migrants to start collecting documentation that would help prove how long they have lived in the country, to ready themselves for the post-Brexit regime.