The Prime Minister’s plan to help young people get on the housing ladder by offering a 20% discount could be blocked unless key changes are made to force buyers to pay back the discount if they sell up.

The decision puts the Government under significant pressure to get the bill through both houses before MPs break in May up ahead of local elections and the EU referendum.

Peers supported two key amendments to the Housing and Planning Bill.

One will force those who buy under the scheme to repay the discount they receive if they sell up, less 5% for every year they own the property over a 20 year period.

The second would allow councils to choose how many starter homes are built in their area in a bid to make sure affordable housing for those on low incomes remains a priority.

Brandon Lewis, the housing minister, said the Government’s commitment to giving first-time buyers a 20% discount on new developments is “unwavering”.

He added: ” The Government believes it is wrong that a 30-year-old couple’s aspirations should be thwarted by having to wait until they are 50 to benefit from the full value of their starter home.”

But Lord Kerslake, the former head of the civil service, said the changes would make the bill fairer and protect the taxpayer by ensuring people who buy starter homes repay the discount when they move.

The Government was also forced to make a last-minute concession to avoid a third defeat, promising that safeguards would be brought forward for so-called rural exception sites to ensure that starter homes will not be built on land earmarked for affordable housing.”