The Railways Act of 1921 forced the merger of the Midland with the London and North Western Railway (L&NWR) into the London, Midland and Scottish Railway (LMS), and the LMS adopted the LNWR's (the "Premier Line") Euston station as its principal London terminus.
The Midland Grand Hotel was closed in 1935, and the building was subsequently used as offices for British Railways.
During World War II, bombing inflicted damage on the train shed, which was only partially reglazed after the war.
On the night of 10–11 May 1941 a bomb fell onto the station floor at platform 3, exploding in the beer vaults underneath.
The station was not significantly damaged, but was closed for eight days, with platforms 2–3 remaining closed until June.
In 1947 the St. Pancras junction was relaid with prefabricated trackwork, along with associated changes to the signalling system.

On the creation of British Railways (BR) in 1948, St Pancras received a significant investment after neglect by the LMS.
Destinations included the London area services to North Woolwich, St Albans and Bedford.
Long-distance trains reached Glasgow, Leeds, Nottingham, Sheffield and Manchester, with famous named trains including The Palatine to Manchester, The Thames-Clyde Express to Glasgow, and The Master Cutler to Sheffield (transferred from King's Cross in 1966, which itself had transferred from Marylebone eight years earlier).

On 7 October 1957, the signalling at St Pancras was upgraded, replacing the three original boxes with a power box controlling 205 route switches and 33 points over a network of 1,400 relays.
From 1960 to 1966, electrification work on the West Coast Main Line between London and Manchester saw a new Midland Pullman from Manchester to St Pancras.
These trains and those to Glasgow were withdrawn following the completion of the rebuilding of Euston and the consolidation of these services.

By the 1960s, St Pancras was seen as redundant, and several attempts were made to close it and demolish the hotel (by then known as St Pancras Chambers).
These attempts provoked strong and successful opposition, with the campaign led by the later Poet Laureate, John Betjeman.
Jane Hughes Fawcett with the Victorian Society was instrumental in its preservation, and was dubbed "the furious Mrs. Fawcett" by British rail officials.
Many of the demonstrators had witnessed the demolition of the nearby Euston Arch a few years previously, and were strongly opposed to the distinctive architecture of St Pancras suffering the same fate.
The station became Grade I listed building in November 1967, preventing any drastic modifications.
The plans were scrapped by BR in December 1968, realising that it was more cost-effective to modernise the hotel instead, though they disliked owning it.

In the 1970s, the train shed roof was in danger of collapse, and the newly appointed Director of Environment Bernard Kaukas persuaded the company to invest £3m to save it.
In 1978, a Private Eye piece said that British Rail really wanted to demolish St Pancras but were opposed by "a lot of long-haired sentimentalists" and "faceless bureaucrats" and praised the office blocks that replaced the Euston Arch.

After the sectorisation of British Rail in 1986, main-line services to the East Midlands were provided by the InterCity sector, with suburban services to St Albans, Luton and Bedford by Network SouthEast.
In 1988 the Snow Hill tunnel re-opened resulting in the creation of the Thameslink route and the resultant diversion of the majority of suburban trains to the new route.
The station continued to be served by trains running on the Midland main line to Leicester, Nottingham and Sheffield, together with a few suburban services to Bedford and Luton.
These constituted only a few trains an hour and left the station underused.

Following the privatisation of British Rail, the long-distance services from St Pancras were franchised to Midland Mainline, a train operating company owned by National Express, starting on 28 April 1996.
The few remaining suburban trains still operating into St Pancras were operate
The Railways Act of 1921 forced the merger of the Midland with the London and North Western Railway (L&NWR) into the London, Midland and Scottish Railway (LMS), and the LMS adopted the LNWR's (the "Premier Line") Euston station as its principal London terminus. The Midland Grand Hotel was closed in 1935, and the building was subsequently used as offices for British Railways. During World War II, bombing inflicted damage on the train shed, which was only partially reglazed after the war. On the night of 10–11 May 1941 a bomb fell onto the station floor at platform 3, exploding in the beer vaults underneath. The station was not significantly damaged, but was closed for eight days, with platforms 2–3 remaining closed until June. In 1947 the St. Pancras junction was relaid with prefabricated trackwork, along with associated changes to the signalling system.
On the creation of British Railways (BR) in 1948, St Pancras received a significant investment after neglect by the LMS. Destinations included the London area services to North Woolwich, St Albans and Bedford. Long-distance trains reached Glasgow, Leeds, Nottingham, Sheffield and Manchester, with famous named trains including The Palatine to Manchester, The Thames-Clyde Express to Glasgow, and The Master Cutler to Sheffield (transferred from King's Cross in 1966, which itself had transferred from Marylebone eight years earlier).
On 7 October 1957, the signalling at St Pancras was upgraded, replacing the three original boxes with a power box controlling 205 route switches and 33 points over a network of 1,400 relays. From 1960 to 1966, electrification work on the West Coast Main Line between London and Manchester saw a new Midland Pullman from Manchester to St Pancras. These trains and those to Glasgow were withdrawn following the completion of the rebuilding of Euston and the consolidation of these services.
By the 1960s, St Pancras was seen as redundant, and several attempts were made to close it and demolish the hotel (by then known as St Pancras Chambers). These attempts provoked strong and successful opposition, with the campaign led by the later Poet Laureate, John Betjeman. Jane Hughes Fawcett with the Victorian Society was instrumental in its preservation, and was dubbed "the furious Mrs. Fawcett" by British rail officials. Many of the demonstrators had witnessed the demolition of the nearby Euston Arch a few years previously, and were strongly opposed to the distinctive architecture of St Pancras suffering the same fate. The station became Grade I listed building in November 1967, preventing any drastic modifications. The plans were scrapped by BR in December 1968, realising that it was more cost-effective to modernise the hotel instead, though they disliked owning it.
In the 1970s, the train shed roof was in danger of collapse, and the newly appointed Director of Environment Bernard Kaukas persuaded the company to invest £3m to save it. In 1978, a Private Eye piece said that British Rail really wanted to demolish St Pancras but were opposed by "a lot of long-haired sentimentalists" and "faceless bureaucrats" and praised the office blocks that replaced the Euston Arch.
After the sectorisation of British Rail in 1986, main-line services to the East Midlands were provided by the InterCity sector, with suburban services to St Albans, Luton and Bedford by Network SouthEast. In 1988 the Snow Hill tunnel re-opened resulting in the creation of the Thameslink route and the resultant diversion of the majority of suburban trains to the new route. The station continued to be served by trains running on the Midland main line to Leicester, Nottingham and Sheffield, together with a few suburban services to Bedford and Luton. These constituted only a few trains an hour and left the station underused.
Following the privatisation of British Rail, the long-distance services from St Pancras were franchised to Midland Mainline, a train operating company owned by National Express, starting on 28 April 1996. The few remaining suburban trains still operating into St Pancras were operate
St Pancras railway station