Farron, the strongly pro-Remain leader of the centre-left Liberal Democrat Party, the UK's third largest, declared shortly after the referendum that his party would base its next election campaign on halting Brexit and keeping Britain in the EU.
Now the Lib Dem MP has reached out to what he called his 'liberal colleagues' to ask for their support in protecting the interests of overseas EU citizens.
Farron has also called for the British government - which sees its leadership change hands from David Cameron to Theresa May on Wednesday - to agree that EU nationals should be allowed to remain in the UK following the referendum.
The Liberal Democrats will next week propose a bill aiming to offer such protection to EU citizens living in the UK, the party announced on its website on Tuesday.
In the open letter, Farron calls for Europe's liberal leaders for their support in 'campaigning for the rights of all British citizens who are residing in Europe to [have the right to stay] in the same circumstances' as EU citizens in Britain.
“Brits working and living abroad are worried about what comes next, I am calling on these Prime Ministers to join me and offer a clear plan that nothing needs to change. I believe our nations are better and richer for this diversity and I am calling on my liberal colleagues to keep making the case,” said Farron in a statement on the Lib Dems' website.
As well as Rasmussen, Farron addressed the open letter to six other European leaders including Mark Rutte of the Netherlands, who also currently holds the EU's rotating presidency. The open letter was additionally sent to the prime ministers of Belgium, Finland, Slovenia, Estonia and Luxembourg.
The political unrest across the North Sea has seen a swell in Danish support for the EU following June's Brexit vote.
Farron's statement and letter in full can be found here.