The independent watchdog responsible for the government’s economic forecasts has blamed the eurozone crisis and high oil prices for its failure to foresee a £58bn overshoot in state borrowing during George Osborne’s tenure as chancellor.

In an assessment of its own forecasting record, the Office for Budget Responsibility (OBR) said much weaker than expected tax revenues had resulted in a budget deficit of £76bn in 2015-16, £58bn bigger than it had predicted when the coalition government came to office in 2010.

The OBR said it did not believe the austerity policies implemented by Osborne when he was at the Treasury were the reason for the forecasting error because it had known that fiscal policy – decisions related to tax and spending – would be tougher from 2010 onwards.

“We consider the more plausible explanation for the pattern of our growth errors across recent years to be the deterioration and subsequent improvement in confidence and credit conditions, largely associated with the sovereign debt crisis in the euro area,” the OBR said in its 2016 forecast evaluation report.

“The oil-price driven rise in inflation in 2011 that reduced households’ real incomes was also an important explanation at the start of the period.”

The OBR said the deficit in 2015-16 had been higher than forecast despite four factors that had been helpful in reducing borrowing: debt interest payments that were £23bn lower than predicted; a £6bn reduction in expected welfare spending; and lower spending on running costs and investment by Whitehall departments worth a combined £1.2bn.

However, these windfalls for the Treasury were more than offset by weaker tax receipts, which were £85bn lower in 2015-16 than the OBR anticipated in 2010.

The OBR said £70bn of the shortfall in receipts was due to weaker growth, with the remaining £15bn caused by the fact that the state was getting less tax for every pound of economic output. With employment growth concentrated in low-pay sectors, the exchequer has not been receiving the income tax and national insurance contributions that were expected six years ago.

Osborne set up the OBR in one of his first moves as chancellor, saying he wanted to remove the risk of political interference from forecasts for the economy and for government finances.

But the slowdown in the economy during the first half of the last parliament caught the OBR unawares, it admitted.

“Over the five years of our June 2010 forecast, real GDP growth has been weaker than expected, with the shortfall focused in the first half of that period. In expenditure terms, the error can be explained by weaker-than-expected investment and net trade.”

The evaluation report added: “The continued weakness in productivity growth has been very unusual by historical standards and is responsible for a large part of our error in forecasting real GDP growth. This error was only partly offset by stronger-than-expected growth in employment.”

In a separate working paper, the OBR said the government had lost hundreds of millions of pounds in revenue by pre-announcing changes to property taxes such as stamp duty, giving nimble homebuyers time to speed up or delay transactions to take advantage of lower rates.

It said by far the most expensive of the six separate incidents studied was the November 2015 announcement of a 3% surcharge for second-home buyers, which was not implemented until March 2016.

“The latest data suggest that around 40,000 transactions were brought forward to beat this increase, with an average tax-saving of £7,560 and a total cost to government of around £300 million. This was a considerably larger effect than the OBR had assumed when forecasting its impact.”

The OBR said the four-month gap gave buyers and sellers plenty of time to bring forward their transactions, noting that even when the window was as short as 11 hours, some buyers were still fleet-footed enough to beat the tax authorities.

It said Osborne had announced changes to stamp duty in his autumn statement in 2014 that reduced the tax on sales over £937,500 but reduced it on all cheaper properties. Although potential buyers only had 11 hours to finalise deals, four times as many transactions took place than usual, costing the exchequer £5m and saving someone buying a £1.5m property £92,000 on average.