If the swing constituency of the Cities of London and Westminster is won by a single vote, it may all be down to one man who was not even eligible to vote himself.

Moritz Hauschulz, a German citizen living in London, was about to board a Eurostar train at the Gare du Nord in Paris on Thursday morning when he was approached by stranger clutching an envelope.

The stranger, Lawrence Cheung, explained that he was a Briton living in Paris who had received his postal ballot too late and needed someone travelling to London to deliver it to a polling station there. Hauschulz agreed, and several hours later, managed to cast Cheung’s vote at a polling station in the Barbican, in what could prove to be a pivotal act in this closely fought constituency.

Hauschulz told the Guardian: “It is unusual to be approached by a stranger in a train station like that, but I tend to give people a listen to see what they’re asking for. It didn’t take long for me to understand what he was trying to do – not least because as a German I have been in a similar position before, when I was living in the US.”

Cheung said: “Moritz took my telephone number and promised he would message if he had a problem, or if he managed to deliver my vote. That’s what he did: he buzzed me by WhatsApp two hours later with a photo saying he had delivered it – I’m very grateful and relieved.”

This election has been beset by problems with postal voting, with widespread reports of people disfranchised because their ballot papers arrived too late. The problems have been blamed on a range of factors, including people going on summer holiday before their papers arrive and errors in the printing process.

Since one in five voters cast their ballots by mail, the problems have the potential to delay the results in some seats and even affect the outcome in parts of the country. The Election Commission has said it will investigate fully once the election is over.

Cheung’s ballot arrived at his Paris home the day before the election, too late for him to return it in time. He said the problem arose more as a result of his failure to find the correct documents to apply for the vote than because of any administrative errors by UK authorities or postal services.

Luckily for Cheung, he spotted a note on the front of his ballot form saying he or someone else could hand in his postal ballot to a polling station in his central London constituency. The only problem was, he was not due to travel back to London on Thursday, and did not know anyone else who was.

So Cheung did the only thing he could: he travelled across Paris to the Gare du Nord and tried to find a friendly Londoner who might take his vote back for him.

After around an hour of fruitlessly approaching strangers, Cheung happened across Hauschulz, who happily agreed, explaining that he had once been unable to cast a postal vote in a German election for similar reasons.

Hauschulz did not even ask which party he was helping by taking Cheung’s vote with him across the Channel. “I wouldn’t have wanted to know how he was voting,” Hauschulz said. “I didn’t want it to subconsciously affect me.”

After a brief delay while Hauschulz tried to explain to some baffled election officials over the phone what he was trying to do, he made his way to the Barbican and successfully cast Cheung’s vote there. Next time, he is hoping he might be able to do the same, but with his own ballot.

“I am planning to apply for citizenship next year, so hopefully I will be able to vote here soon,” Hauschulz said. “This time, at least, I was able to help the process.”

• Postal voting
• General election 2024
• General elections
• London
• Paris
• France
• Eurostar
• news