Britain's economy could have £40billion wiped off growth in just two years by a no-deal Brexit, a major international report warned today.

The Organisation for Economic Co-operation and Development (OECD) said the Government should look to maintain the 'closest possible' relationship with the EU to avoid a downturn.

Its research said failure to reach a Brexit deal could wipe off more than 2 per cent from real gross domestic product over two years - roughly £40billion.

The OECD forecast comes just a day after Bank governor Mark Carney warned a no deal Brexit would be the biggest hit to the economy since the 1970s.

Britain economy could have £40billion wiped off its value by a no-deal Brexit scenario, a major international report warned Theresa May (pictured today at PMQs)

In its latest report, the OECD upped its forecast for UK growth next year to 1.4 per cent from its previous prediction of 1.1 per cent.

It maintained its forecast for growth of 1.3 per cent in 2018.

But the OECD said growth was then set to slow markedly to 1.1 per cent in 2020, while its warnings on the impact of Brexit are stark.

It said: 'The failure to come to a withdrawal agreement with the European Union is by far the greatest risk in the short term.'

It added: 'The lack of details on the future relationship between the UK and the EU or the extension of the transition period, and the resulting uncertainties, could incite businesses to delay investment plans further.

'By contrast, prospects of maintaining the closest possible economic relationship with the European Union would lead to stronger-than-expected economic growth.'

The OECD also gave a more gloomy assessment of the global economic outlook, downgrading its 2018 and 2019 forecasts for growth in the G20 group of economies to 3.8 per cent and 3.7 per cent respectively.

The OECD forecast comes just a day after Bank governor Mark Carney (pictured in Parliament yesterday) warned a no deal Brexit would be the biggest hit to the economy since the 1970s

It predicts overall world growth will remain at 3.7 per cent in 2018 and downgraded it to 3.5 per cent next year, with a particular warning over the mounting trade tensions between the US and China.

Angel Gurria, OECD secretary-general, said: 'Trade conflicts and political uncertainty are adding to the difficulties governments face in ensuring that economic growth remains strong, sustainable and inclusive.'

On slowing global growth, chief economist Laurence Boone added: 'There are few indications at present that the slowdown will be more severe than projected.

'But the risks are high enough to raise the alarm and prepare for any storms ahead. Co-operation on fiscal policy at the global and euro level will be needed.'