"We are no closer to knowing the relationship Britain is going to have (with the EU)", said Jane Foley, a currencies strategist at Rabobank, adding that many investors were waiting it out in the meantime.
Flanders' exports to Britain are worth some 28 billion euros (24.5 billion pounds) per year and the Flemish government said sales to Britain could shrink up to 2.5 percent in case of a hard Brexit.
"We respect Britain's red lines". 45% of those polled in the province said they wanted to remain in the UK.
Foreign Secretary Boris Johnson has complained that the Irish border issue is dictating "the whole of our agenda" in the UK's Brexit negotiations with the EU.
May refused to give a "cast-iron guarantee" that the cut-off date for the backstop plan would not extend beyond December 2021, but said that the government expects to have a better customs arrangement in place by then "at the very latest".
The Irish government - which is pressing for "substantial progress" on the issue by an European Union summit later this month - said it would consider the proposals.
Over the past 24 hours there were veiled threats that the Secretary of State for Exiting the EU, David Davis, might resign if a deadline was not set on the extension of customs arrangements.
The proposed United Kingdom backstop "will only be in place until the future customs arrangement can be introduced", which the government "expects" to be the end of December 2021 at the latest. But pro-EU legislators from both the Conservative and Labour parties say they will push for the U.K.to remain inside the single market by joining the European Economic Area, a grouping of the EU plus Norway, Iceland and Lichtenstein.
The EU proposed keeping Northern Ireland in the customs union but this was rejected by the government as well as Northern Irish party the DUP, which is holding together Mrs May's minority government.
Meanwhile Tory Brexit rebels were seen in Downing Street as the Prime Minister sought to avert defeat when the EU (Withdrawal) Bill returns to the Commons on Tuesday. "A people's vote must be held before March 29, 2019, to decide our country's future", the organization said in its roadmap.
The Best For Britain group plans to secure a second referendum via an amendment to the EU Withdrawal Agreement and Implementation Bill, which will be discussed and voted in the House of Commons in autumn.
"It would tie us effectively into the EU's customs arrangement for an indefinite period", he said.