Boris Johnson’s claims to be conducting a Brexit renegotiation have come under fresh doubt as the UK’s closest EU ally expressed alarm at the “breathtaking” developments in Westminster and the government’s failure to table alternatives to the Irish backstop.

The Dutch minister for trade, Sigrid Kaag, described the scenes being played out in London as “unbelievable” and warned of the EU’s waning patience given the impact on European businesses of the continued uncertainty.

Johnson committed in early August in discussions with EU leaders to swiftly tabling an alternative solution to the Irish backstop, which would temporarily keep Northern Ireland in the single market and the UK in a customs union to avoid a hard border on the island of Ireland.

The lack of such proposals and the attempt by the Brexit secretary, Stephen Barclay, over the weekend to lay the blame for a lack of progress on the European commission has confirmed to many in Brussels that the outgoing work and pensions secretary, Amber Rudd, was right about the prime minister’s lack of seriousness in sealing a deal.

In an interview with the Dutch financial newspaper Het Financieele Dagblad, Kaag warned that given the damage caused to European businesses, rebel MPs seeking to delay Brexit would need to tie it to a general election or second referendum to be sure of securing an extension of UK membership of the EU beyond 31 October.

“At a point enough is enough,” Kaag said. “At that stage the certainty of deterioration might be better than continuing without perspective. At a certain point there must be clarity. A good reason is needed for a new delay. What? It is difficult to say categorically.

“You have to see the context and see if there is a realistic expectation that this will lead to a follow-up phase. But so far, the British have not presented an alternative to the Brexit agreement that is already in place.”

Kaag was speaking as figures from The Hague revealed companies from the United Kingdom had substantially increased their investments in the Netherlands in the last year, to a total of €80bn compared with €14bn in 2016.

The UK is now the largest investor in the Netherlands. In contrast, Dutch companies divested €11bn over the last year, compared with an investment of €50bn in 2016.

“The business case for Brexit is still not well-founded,” Kaag said. “People cast their votes based on an emotion or assessment without socio-economic grounds. The evidence was ignored. The damage is now clear. I think it’s dramatic.”

The minister warned that the EU’s patience over the UK’s prevarication would not last forever given the impact on the bloc’s ability to steer its own way forward.

Kaag said: “It is always better if you have your house in order and can say: now we are going to get involved with the Americans or prepare ourselves better for the negative consequences of the trade war between the US and China. Brexit always detracts from that.”