Brighton Lib Dems and Greens in party alliance
The Liberal Democrats and Green Party have said they will not stand against each other in two Brighton constituencies in the general election.
The Lib Dems said they would not contest the Brighton Pavilion seat held by the Green's only MP, Caroline Lucas.
The Greens said they would not be fielding a candidate in Conservative-held seat of Brighton Kemptown.
Ms Lucas said: "People are putting aside party allegiances and working together."
Sussex's only Labour MP, Peter Kyle, has already ruled out an electoral pact with the Greens.
The Lib Dems and Green Party will both field candidates in his Hove constituency.
In Brighton Pavilion, Ms Lucas retained the seat in 2015 with a majority of almost 8,000 over Labour. The Lib Dems finished fifth.
Brighton Lib Dems tweeted: "Our Brighton Pavilion constituency members voted tonight for the #LibDems to stand down in favour of @CarolineLucas in #GeneralElection2017."
In Brighton Kemptown, Davy Jones, who had been selected as the Green Party's parliamentary candidate, tweeted: "Delighted that @BHGreens [Brighton and Hove Green Party] decided we unilaterally stand down in Brighton Kemptown to help defeat sitting Tory MP."
In 2015, the seat was defended by Simon Kirby with a majority of 690 over Labour. The Greens came fourth behind UKIP, pushing the Lib Dems into fifth place.
Lib Dem president Baroness Brinton said: "In Brighton Pavilion, local Liberal Democrats have decided to pursue [the] challenge jointly with the local Green Party.
"We welcome that constructive collaboration."
Ms Lucas said: "Something amazing is happening in Brighton.
"People are putting aside party allegiances and working together. Members of local parties are taking brave decisions for the common good."
Analysis: Ben Weisz, political reporter, BBC Sussex
You've heard of tactical voting - at this election, a degree of tactical standing will be at play, nowhere more so than in Sussex.
In Kemptown, if everyone who voted Green in 2015 had voted Labour instead, Nancy Platts would be the MP, not Tory Simon Kirby. If voters hadn't had the option of voting Green, Labour might have won. That's the theory.
In practice, it's a little more complicated. Can we assume Simon Kirby and the Tories haven't won over Labour voters over the last two years? Can we assume Green voters would naturally flock to Labour? Can we assume first-time voters aren't going to turn out for other parties and spoil the maths?
And the loose arrangements emerging in Sussex are far from complete. The Lib Dems have stood aside to help Caroline Lucas - but so far the Greens haven't decided whether to return the favour and stand aside in Lewes, a key Lib Dem target.
"Alliances", "pacts" - whatever you call them, these moves have certainly changed the electoral balance in Brighton. But there's still plenty to play for, and no guarantee they'll work.