UK prime minister David Cameron's election victory comes with the price of an in/out EU referendum. MEPs have raised their concerns at the possibility of a so-called 'Brexit'.
The European parliament's S&D group leader Gianni Pittella called the result "surprising and disappointing" for the Social Democrats, urging UK prime minister David Cameron to "make a strong and positive case for Britain's continued [EU] membership".
He added, "we are willing to work with the new government but we will not make any concessions in regard to the fundamental principles on which the EU was founded".
His Belgian colleague Marc Tarabella tweeted, "Cameron's populist politics are a danger to the future of Britain and its people".
Meanwhile, parliament's Greens/EFA group co-chairs Rebecca Harms and Philippe Lamberts warned that Cameron's victory "brings with it an enormous responsibility for the direction of the UK, its relationship with Europe and the future of the European Union".
They stressed that, "the lack of political leadership on Europe has led us to a situation in which the UK is sleepwalking its way out of the EU. This would have dramatic and negative consequences for the UK and its component nations, as well as for the rest of Europe.
"We can only hope David Cameron finally wakes up to this risk and mobilises as wide a coalition as possible to campaign in favour of EU membership and against the hara-kiri of Brexit".
However, over on the centre-right, parliament's EPP group leader Manfred Weber was slightly more nuanced in his comments, saying, "on the referendum, the ball is in Cameron's court. He has to put his demands on the table, but EU freedoms will not be negotiable".
Weber also noted, "we Europeans must also start thinking about whether it is time for a larger treaty reform".
Unsurprisingly, parliament's ECR group leader Syed Kamall - himself a member of the UK's Conservative party - called the outcome, "an excellent result […] that has confounded all expectations".
Pollsters had predicted a hung parliament, with no party gaining an overall majority.
Kamall said his group "will work closely with [Cameron] and his team as they seek not just a better deal for Britain but a more flexible, open and eurorealist EU that will deliver a better deal for all of its members".
He added, "this has been a good weekend for the ECR with our member parties in the UK and Finland looking set to enter government and a likely strong showing in Sunday's presidential election in Poland. Clearly the cause of EU reform continues to gain ground right across the EU".
It was announced last week that Finland's likely next prime minister, ALDE-affiliated Juha Sipila, would form a coalition with the Eurosceptic Finns party, a member of the ECR group.