LONDON — The economic forecasts at the heart of Chancellor Philip Hammond's Budget speech are based on the assumption that annual net immigration to Britain will not drop below 100,000 once Britain leaves the European Union.

Hammond used his first budget to drive home the Conservative government's message that the national economy remains stable and resilient despite the uncertainty caused by Britain's vote to leave the European Union.

What Hammond didn't mention in his speech to MPs was that the Office for Budget Responsibility's projections were based on the assumption that Brexit will not reduce immigration to the extent that some pro-Leavers had hoped and predicted.

The key line can be found on page 33 of the OBR's 'Economic and fiscal outlook' under the sub-heading 'assumptions regarding the UK’s exit from the EU' — here it is:

The OBR add that their forecasts are based on the assumption that at most, immigration will fall to 185,000 by 2021.

Bringing net migration to the UK below 100,000 people a year was originally a pledge made by the Tories under former Prime Minister David Cameron in the run-up to the 2010 general election. The party decided to stick with the ambitious policy despite the obvious hurdle posed by the free movement of people within the European Union.

Net migration to the UK was 273,000 last year, according to the latest Office for National Statistics research.

One of the key reasons put forward by Brexit campaigners for why the country should leave the 28-nation bloc was that Britain could "take back control" of its borders and substantially reduce the number of people entering the country year on year.

Vote Leave suggested that an Australian-style points system would be an effective means of bringing net migration down to the tens of thousands once Britain had formally left the EU.

Brexit secretary David Davis said during the campaign that net migration to the UK could fall to almost zero, if Britain voted to leave the EU.

Here's everything else you need to know about the Spring Budget.