Crashing out of the EU at Halloween could result in a “flurry of profit warnings” from publicly listed companies in November, MPs have been warned.

The Institute for Chartered Accountants for England and Wales (ICAEW) told MPs that this could have a “systemic” impact on the confidence in the British economy.

“I would like to have this on record, it is likely we may well see after November 1 a flurry of profit warnings from companies finding themselves in completely unprecedented circumstances,” Martin Manuzi, the regional director for Europe of the ICAEW told the Brexit select committee.

The ICAEW has about 100,000 members, including the big accountancy firms, which provide auditing for the UK’s biggest companies.

Manuzi said if there was a sudden “dislocation” in the market, public limited companies would be obliged to tell their shareholders.

“Things that we ask ourselves? What is the cumulative effect on market confidence of the issuing of such profit warnings, and a systemic lack of confidence can have massive macroeconomic impacts,” he said.

Giles Derrington, the head of Brexit policy at techUK, told the committee that the decline in investment in startups was having a damaging impact on the UK’s reputation as the obvious place for tech outside the US.

He said in the first three months of 2019 venture capital investment in the UK had fallen by “about 58% in terms of the deals done” and Germany was now catching up with the UK, which had been the clear frontrunner in the sector before.

The Brexit committee was hearing evidence about the impact of Brexit on the services sector, which accounts for 80% of the UK’s economy.

Manuzi denied his statements were part of “project fear” when challenged by two MPs.

“I’m not here to do scaremongering,” he said. His members had ‘“very deep concerns” about a loss of access to EU markets and it was very likely companies would be “looking again at what their profit forecasts will be and they may well be issuing statements” if there was no Brexit deal.

Manuzi’s comments came a day after Boris Johnson said the UK would quit the EU on 31 October “do or die” if he became prime minister.

Manuzi said his members had been clear they did did not want a disorderly Brexit and still hoped the new prime minister would not plunge the UK into a no-deal departure.

MPs also heard that Brexit uncertainty was causing “reputational impact” on tech investment, one of the UK’s flagship sectors.

Derrington told the committee that a no-deal outcome would have “significant impact” on data flows, the ability to recruit talented people and “wider perceptional reputation damage to the UK as a place for the global tech centre to come and grow and develop”.

“UK might be the best place to invest, but it is no longer the only place to invest,” he said.

He said established firms were getting their funding but it was the “new, more risky innovative startups” that were vulnerable.

This alone was a concern as to attract tech investors, a country had to have the entire “ecosystem” of “very large multinationals, strong UK mid-tiers, successful growing firms and exciting startups working in tandem”.

“So if you lose one, the longer-term impact of that can cascade over five, 10 years.

“In an industry that moves very, very quickly, it’s then very hard to catch up,” he said.

He said France and Germany were making a big play for Brexit dividends in the tech centre and this was contributing to a decline in investment in the UK.

In 2018, the UK had “almost double” the tech venture capital of Germany, but year on year there had been a 20% decline for Britain and Germany had a 30% increase.

“The worry is that gap sufficiently closing that we are no longer the obvious global hub outside America,” he said.