Premier League clubs have already spent more than £1.5bn on new signings across this season's two transfer windows - although they are currently some way short of the £180m spent during the January window last season.
Clubs across Europe, including in England and Scotland, cannot sign players after Friday, 31 January.
For Premier League and English Football League clubs the deadline is 23:00 GMT - with Scotland's at midnight.
Some reported transfers include Manchester United's pursuit of a striker - with Monaco's on-loan Leicester forward Islam Slimani being considered - and Arsenal's links with Atletico Madrid winger Thomas Lemar.
What else do you need to know about deadline day?
Can deals be completed after the deadline?
In the Premier League, clubs completing deals late on deadline day will be able to submit a 'deal sheet' between 21:00 and 23:00 GMT in order to give them more time to complete a transfer.
Clubs will then have until midnight on 31 January to complete the information required by Fifa's transfer matching system to apply for international clearance for new signings.
What about loans and out-of-contract players?
Clubs can sign free agents after the deadline - at any stage of the season - so long as they left their previous club before the transfer deadline.
No loan signings can be made by EFL or Premier League clubs after the deadline, except in extreme circumstances - such as a club's senior goalkeepers all being unavailable.
How about other countries?
The transfer deadline is on Friday in all of Europe's major leagues. German Bundesliga clubs have until 17:00 GMT, with Italy's deadline at 19:00 GMT and France and Spain's windows closing at 23:00 GMT.
When can new signings play?
New players can only feature in this weekend's games if they are signed and registered by 12:00 GMT on Friday. Otherwise they will have to wait for the following round of Premier League matches, which is split over the next two weekends.
Nobody can play in next week's FA Cup replays if they were not at the club for the initial fixture.
How much has been spent so far?
Premier League spending in January is expected to fall for the second year in a row.
Top-flight clubs have spent £99.2m so far this month, with Manchester United also set to spend an initial £47m on Bruno Fernandes. Last January clubs spent £180m.
However, Premier League spending for the season so far is over £1.5bn - the second highest total ever.
What are the biggest deals of this window so far?
There have not been many big Premier League signings this month.
Spurs have made the loan signing of Real Betis midfielder Giovani lo Celso permanent for a fee of £27.2m and added forward Steven Bergwijn for £27m from PSV Eindhoven, after selling Christian Eriksen to Inter Milan for £16.9m.
Aston Villa spent £10m on Genk striker Mbwana Samatta and Watford signed Udinese winger Ignacio Pussetto for £7m.
Elsewhere, Real Madrid spent £26m on Flamengo youngster Reinier Jesus and Borussia Dortmund bought Red Bull Salzburg wonderkid Erling Braut Haaland for £17m.
What are the biggest January deals ever?
The two biggest January transfers ever happened in 2018 and both involved Liverpool.
They sold Philippe Coutinho to Barcelona for a fee which could yet rise to £142m and spent £75m on Southampton centre-back Virgil van Dijk.
The next biggest January signings by English clubs have been Chelsea's £58m recruitment of Borussia Dortmund winger Christian Pulisic last year - although he stayed with Dortmund on loan - and Manchester City buying Athletic Bilbao defender Aymeric Laporte for £57m in 2018.