The leaders were in danger of falling to their first defeat in 10 matches with five minutes remaining, trailing 2-1 against a Millwall side who have not won at home since Boxing Day.
But Paul Robinson, whose volley put Millwall ahead after Jose Fonte's own goal had cancelled out Lambert's opener, fouled the striker in the area for his first spot-kick and three minutes later Alan Dunne was adjudged to have handled and Lambert helped himself to another treble.
Brian McDermott's Reading had won eight games on the trot before drawing at Doncaster in midweek, but two goals in as many minutes at the start of the second half got them back on the winning trail before their dominance was confirmed with two further efforts.
When Luciano Becchio nodded Leeds ahead with seven minutes to go, the ever-changing npower Championship promotion picture was not looking too favourable for the Hammers, with top two rivals Southampton and Reading both winning.
But, right on cue, Collins rose the highest to head in a Mark Noble corner and peg Leeds back in an act of damage limitation for Sam Allardyce's men, who are now five points off the lead and three points shy of second.
Birmingham marked their 50th match of a long season with a welcome 3-0 victory over promotion rivals Middlesbrough that saw them move back up to fourth place in the Championship.
Much was owed to winger Chris Burke, this week recognised by second-tier managers as one of the top 10 players in the division, who claimed two assists to take his tally to 13 this term - the highest of any Championship player.
The Scot's 11th-minute cross set up Nikola Zigic, whose low header produced his ninth goal of the season and set the hosts on their way to a first win in six games in all competitions since losing their 15-match unbeaten record.
Boro, despite just one win at St Andrew's in 33 years, remained in the game and posed a threat having claimed more points on their travels than at the Riverside this season - Joe Bennett seeing a neat finish ruled out by an assistant's late flag.
That threat was extinguished however thanks to two goals in three minutes around the hour mark, Burke setting up Marlon King for a well-taken strike before man of the match Keith Fahey capped a marauding run with a composed finish.
Kevin Phillips claimed a goal in each half as Blackpool reignited their Championship play-off bid with a 3-1 win against 10-man Brighton.
Joe Mattock headed the visitors into an early lead from Vincente's corner, but centre-back Ian Evatt joined the Seasiders attack in the 37th minute to level matters.
In a frantic period before half-time Blackpool went ahead as veteran striker Phillips demonstrated his poacher's instincts to be as strong as ever.
Phillips knew very little about his second effort - a 15th this term - as Gary Taylor-Fletcher's shot ricocheted off him and beyond Seagulls goalkeeper Peter Brezovan with 10 minutes remaining.
The win lifts Blackpool back into the top six at the expense of Brighton, who are a point behind Middlesbrough in sixth after their first league defeat of 2012.
Nick Barmby's side have now kept 18 clean sheets in the Championship this season, more than any other team in the division.
Gareth Roberts' first goal for Derby piled the misery on his former club Doncaster, whose Championship survival hopes were dealt a blow with a 2-1 home defeat.
Roberts, who joined the Rams from Rovers in May 2010, doubled Derby's lead 10 minutes into the second half after Theo Robinson had put the visitors ahead in the 13th minute.
El-Hadji Diouf pulled a goal back for Doncaster with 23 minutes remaining but Dean Saunders' side could not save their five-match unbeaten streak and slipped four points adrift of fourth-bottom Bristol City after their goalless draw at Portsmouth.
Joe Murphy produced a string of fine saves as relegation-threatened Coventry denied Watford all three points in a 0-0 draw at Vicarage Road.
John Eustace and Troy Deeney had the best of the chances but Murphy stood firm as Coventry's search for a first Championship away win of the season continues.
Aaron Cresswell crashed in a winner five minutes from time as Ipswich recorded their fourth successive home win by beating Peterborough 3-2 in a pulsating contest.
The hosts, who had added motivation after Darren Ferguson's side inflicted a 7-1 hammering in the reverse fixture, led when midfielder Grant Leadbitter drilled in his third goal in his last two games.
Peterborough fielded eight players who started the drubbing at London Road but it was recent signing Tyrone Barnett who brought them level before half-time.
There were then three goals in the final 13 minutes as the game's end-to-end nature continued right up to the final whistle.
Daryl Murphy's close-range header put Ipswich back in front but moments later substitute Emile Sinclair, who had been on the pitch for just five minutes, squeezed in a leveller. However, just two minutes later full-back Cresswell's long-range shot took a deflection to settle the game in the hosts' favour.
Portsmouth and Bristol City shared the spoils in a 0-0 draw at Fratton Park as both sides spurned second-half chances to secure a vital win Championship relegation battle.