The Queen’s accountants have warned Brexit is a potential risk to her finances as she received another record multimillion-pound surplus from her private estate last year.

The annual report of the Duchy of Lancaster recorded a £20.2m profit in 2017-18, an increase of 4.9% on last year.

The profit from the Queen’s 753-year-old ancestral estate is almost three and half times as large as it was in 2000, when it stood at £5.8m.

Brexit was mentioned once in the 60-page report, under the heading “strategic risk”.

A carefully worded passage said: “Each year the duchy carries out a five-year business plan as well as preparing rolling forecasts for the year ahead.

“As part of this process, a review is undertaken of long-term trends to assess options for continued and ongoing viability of duchy operations – this would include any outcomes from Brexit negotiations.”

Tom Brake, the Liberal Democrat Brexit spokesman, said: “When even the Queen’s estate are having to assess the risks associated with Brexit, you know it is time to call time on this folly. The people must have the final say on the deal and a chance to exit from Brexit.”

The report flagged up that the duchy was potentially at risk of “criminal cyber-attacks”, but this risk was “mitigated by a number of advanced IT safeguards”.

Although the net surplus on the estate increased at almost double the rate of inflation, the 4.9% rise represented a slowdown: in the previous two years, it had risen by almost 8% after two years of increases of more than 13%.

“This has been another positive year for the duchy with strong growth in almost all our business sectors,” the report said.

At 64%, the bulk of the duchy’s income came from commercial activities, the report said. Agriculture made up 18%, followed by financial activities on 10% and residential income at 8%.

The net value of the estate increased by 2.9% to £533.8m. The report pointed out that the duchy is not subject to corporation tax, but the Queen pays tax on the income she receives from the estate.

Duchy officials have resisted opening up the books to the National Audit Office.

Last year, the Paradise Papers revealed millions of pounds of the estate had been invested in a Cayman Islands fund as part of an offshore portfolio.

Our head of state does not need this £20m when people are struggling to put food on the table ​Graham Smith, CEO of Republic

David McClure, the author of Royal Legacy, an investigation into the royal finances, pointed out that the report made no reference to this offshore investment.

“It is as if the Paradise Papers were a mirage and the bad press a bad dream. Her financial advisers should have had some knowledge of the offshore holdings and ought to have questioned whether they were appropriate or could do reputational damage to the monarchy,” he said.

“All the results from the duchy dairy are laid out in the glossy annual report, but some are about as clear as milk and others raise more questions than answers.”

The report suggested management of the investment portfolio was made independent of the duchy.

“The financial portfolio, as well as providing a significant income for the duchy, remains the major source of liquidity. The management of the quoted portfolio has been predominantly placed in the hands of a single manager, Newton Investment Management Limited, which has helped to enhance accountability and reduce administration,” it said.

Graham Smith, the chief executive of Republic, the campaign for an elected head of state, said: “The duchy of Lancaster is not the Queen’s private property but a land portfolio gifted to her by parliament. Our head of state does not need this £20m income when people are struggling to put food on the table and at a time when there is such economic uncertainty.”