LONDON (REUTERS) - Prime Minister Theresa May will say on Friday (Sept 22) that Britain is willing to pay €20 billion (S$32 billion) to the European Union during a post-Brexit transition period, but only if it has access to the bloc's single market, the BBC's political editor reported.

May, who is weakened after losing her party's majority in a June election, is due to make a speech in Italy setting out her vision for future ties with the EU.

Earlier on Thursday, she briefed her top ministers on the speech.

"UK willing to pay 20 billion euros during transition period but only if we have access to single market and some form of customs union," Laura Kuenssberg said on Twitter, citing a government source.

She added that this would not cover long-term liabilities, so the eventual total bill for leaving the bloc could be far higher.