An extra 400 million customs declarations could have to be issued a year under new post-Brexit border controls, it is understood.

The figure, from sources, came as the Government insisted Britain will have the “most secure” border in the world by 2025.

With Britain set to quit the Brexit transition period by the end of the year, the full tracking of vehicle movements will not be in place until next July, it is understood.

Chancellor of the Duchy of Lancaster Michael Gove said business needs to be ready for the UK becoming an independent trading nation for the first time in nearly half a century when the Brexit transition period is scheduled to finish at the end of the year.

More than £700 million is to be spent on building new infrastructure, hiring staff and developing technology to ensure that Britain’s border systems are fully operational when the UK leaves the EU at the end of the year, Mr Gove said.

The Government insists the border operating model it has announced gives “clarity” to businesses.

Mr Gove said: “The publication of the border operating model is an important step which gives business the certainty and direction they need to prepare for the end of the transition period when the UK becomes an independent trading nation for the first time in nearly 50 years.

“We are committed to working closely with businesses and the border industry to help deliver not just a fully operational border at the end of the transition period, but also the world’s most effective and secure border in the world by 2025.”

The £705 million package includes £235 million for staffing and IT systems, and £470 million for port and inland infrastructure to ensure compliance with new customs procedures and controls.

New border infrastructure will be built inland where there is no space at ports, while ports will get one-off financial support to ensure the right infrastructure is in place.

It is understood there will be 10-12 inland sites.

The funding relates only to the implementation of the GB-EU border, and the Government is expected to publish specific guidance and measures for Northern Ireland in the coming weeks.

The package comes ahead of a public information campaign in which guidance will be given to traders and hauliers explaining what they may need to do to prepare for the end of the transition period.

The letter presents a picture of chaos, complacency and confusion right at the top of Government

Estimates for import declarations range from £20 to £56, and £15 to £46 for export declarations.

Government sources said the final costs of completing customs declarations will “vary significantly” and fees are expected to reduce overtime.

The estimated annual increase in the number of declarations on the UK side is 215 million per year, sources said, with the rest on the continent.

Labour’s shadow chancellor of the Duchy of Lancaster Rachel Reeves told the Commons a leaked letter from International Trade Secretary Liz Truss to Mr Gove “presents a picture of chaos”.

Ms Reeves told MPs: “It is vital that businesses and jobs are supported and that the oven-ready deal that the country was promised is delivered upon this year, and yet frankly, many of us are worried about whether the oven was even turned on.

“There have been alarm bells ringing within the Cabinet this last week, expressed by the Trade Secretary in her extraordinary letter to (Mr Gove) and to the Chancellor of the Exchequer written on 8 July.

“The letter presents a picture of chaos, complacency and confusion right at the top of Government.

“Can the minister give the country and indeed his Cabinet colleague reassurance by publishing all relevant delivery plans, land purchases and rental agreements with time scales and risks, and not just for the port of Dover?

“The Labour Party want to see British firms exporting, we do not want to see their goods stuck at ports or indeed in lorry parks.”