Prime Minister Theresa May had taken a hard-line approach on leaving the European Union but a disastrous election result for her Conservative party on June 8 has left London's policy in disarray and her own political future uncertain.
While European leaders try to gauge what to expect from Britain, May is so weakened that her own finance minister and the partners on whom she will rely for her majority, Northern Ireland's Democratic Unionist Party, are giving her public guidance.
But a deadly fire at a tower block in London could delay the announcement of any deal, BBC political reporter Norman Smith said.
It comes as DUP leader Arlene Foster and the party's Westminster leader Nigel Dodds have been having ongoing discussions with Theresa May to strike a deal for them to work a minority government with the Conservatives.
The negotiations have been billed as the most complex in Britain's history as it unravels 44 years of membership and its threat to walk out with no deal in place has anxious European capitals.
Theresa May's promised "generous offer" on European Union citizens' rights at the start of the Brexit talks this week will fall short of protection of the status quo, meaning a hard start to the negotiations, officials in Brussels fear.
Liberal Democrat Layla Moran, who beat Nicola Blackwood to take the seat by 816 votes, was speaking ahead of the talks and nearly one year after the referendum vote to leave the EU.
But pressure was mounting for May to change course on the type of Brexit Britain should pursue.
"We should be protecting jobs, protecting economic growth and protecting prosperity", Hammond said as he arrived for a meeting of European Union finance ministers in Luxembourg.
Brexit Secretary David Davis will demand a "deal like no other in history" as he heads to Brussels for day one of negotiations on the United Kingdom leaving the European Union. Once done, however, Brexit would be hard to reverse.
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A two year legislative programme rather than the usual one also means MPs and peers will get more time to debate Brexit.
Irish nationalist party Sinn Fein said the prospect of a British agreement with the DUP was causing anxiety and fear.
The DUP is believed to be more favorable to a "soft Brexit" that would keep Northern Ireland's border with the Republic of Ireland free-flowing.
A day after France and Germany both said Britain could reverse Brexit if it wanted, Verhofstadt said Britain's budget rebates and opt outs from European Union rules would disappear if it did so. She has really hacked off the parliamentary party for obvious reasons.
Mr Hammond added: "We have got to make sure our border continues to work seamlessly and that's probably the number one challenge for business".
Monday s negotiations are to start in Brussels at 11 am (0900 GMT) with 90 minutes of talks between Barnier and Davis, followed by a working lunch between the pair, and a press conference.
Divisions over Europe helped sink the premierships of Margaret Thatcher, Major and Cameron, and many of her lawmakers and party membership support a sharp break with the EU.
The performance of the British economy could also influence perceptions of Brexit. However, this is not a short term solution and the case could be dragged on for years.
Sounding conciliatory, Britain's Boris Johnson said as he arrived at a meeting with fellow European Union foreign ministers in Luxembourg that he looked forward to "a happy revolution" in relations that would be good for Britain and the rest of Europe.