The claim: Only one new house has been built for every five council houses sold under the right-to-buy policy, not one for one as was promised.

Reality Check verdict: The promise was kept for the first year of the revamped right to buy, but the National Audit Office has warned that acquisitions and building will have to accelerate sharply if the government is to continue to keep that promise.

Labour leader Jeremy Corbyn said at Prime Minister's Questions: "The former prime minister… promised that there would be a one-for-one replacement for every council house that is sold under right to buy."

"The reality is there's only one for every five that are sold."

Theresa May responded: "In relation to the figures on council houses, he is wrong. We have delivered on the one-to-one replacement on the right to buy."

Who is right?

Replacing affordable homes, whether by buying or building, is not an instantaneous process. Finding the money is not straightforward and housing stock availability varies enormously around the country.

Also, the numbers are calculated at a national level so - in theory - houses sold off in Brighton could have been replaced in Stockport. The policy applies only in England.

This table from the House of Commons Library contrasts the number of sales under the right to buy with the number of properties acquired and builds started by local authorities:

At first glance there is a big discrepancy between the number of properties sold and the number gained - more than six times as many have been sold as have been acquired.

However, the government has not committed to replacing all sales. When the system was being overhauled in 2011-12 the government promised to replace only additional sales.

Before the overhaul, about 2,500 homes were being sold anyway and the government has estimated how many would have been built after that without the revamp.

Also, the promise was to replace sold dwellings within three years of the date they were sold.

So, if you look at the first year of sales after the overhaul, you will see that 5,944 homes were sold. Subtract the estimated 2,890 that would have been sold anyway and you get 3,054.

If you add up the new houses started or bought in the next three years you get 4,471, which is clearly better than the one for one promised.

We do not yet know whether the promise has been kept for 2013-14 because a property sold in March 2014, for example, would not have to be replaced until March 2017.

But it is going to start to get difficult to keep the promise. According to the National Audit Office: "To meet the target of replacing the roughly 8,512 homes sold in 2014-15 by the end of 2017-18 would require quarterly housing starts to reach around 2,130, a fivefold increase on recent figures of approximately 420 per quarter."

The House of Commons Library agrees that there is a chasm about to open up in the numbers, unless people suddenly stop buying council houses or large quantities of housing stock are suddenly bought or started.

"The number [of acquisitions] for the next three years would have to total 24,800 for the commitment to be fully met, even if there were no further sales," it said.