It happened 50 years ago when the sides met in the semi-finals of the European Cup Winners' Cup.

The world of football was very different in those days, yet the events of 1971 offer some resonance in 2021.

Last month, Chelsea beat Real Madrid in the Champions League semi-final. In 1971, Chelsea achieved the same result in the Cup Winners' Cup final.

Fifty years ago, City and Chelsea were both at the top table of English football and their rivalry was spread over three competitions.

For FA Cup holders Chelsea, the season began a week earlier than most of their rivals in August 1970, with a 2-1 defeat against league champions Everton in the Charity Shield. Their campaign did not end until almost ten months later.

Chelsea's Stamford Bridge ground was only a few kilometres from the King's Road which had epitomised "Swinging London". Among their earliest celebrity fans was film actor and director Sir Richard Attenborough.

Their European adventure began in northern Greece against Aris Salonika. Defender John Dempsey was sent off in a 1-1 draw but midfielder John Hollins scored with a long range effort to inspire a 5-1 victory in the second leg.

City had also qualified for the Cup Winners' Cup as holders. They had beaten Polish side Gornik Zabrze to set the seal on a golden season which also included the League Cup. All this followed a league title in 1968 and the FA Cup in 1969.

Northern Irish club Linfield provided their first round opposition in Europe but City went through only on away goals.

That same day, the Hungarian club Honved won the first European tie decided by a penalty shoot-out. They beat Aberdeen although there was a 12 minute delay because of "considerable doubt about the procedure".

There was considerably less doubt when Honved met City in the next round.

Francis Lee's goal gave City a win in Budapest, before a 2-0 victory at Maine Road to seal their passage to the quarter-finals. Chelsea also progressed against the Bulgarian side CSKA Sofia.

The quarter-final draw was made in November but there was then a break until the competition resumed in March.

In the meantime, English domestic football continued and, just as in 2021, Chelsea and City met in the FA Cup.

Holders Chelsea were at home for the fourth round tie played at the end of January.

"It is a plum draw for the fans but most players must have wished that a meeting like this could have been left for a little later in the tournament," the Kensington Post in West London told its readers.

Rain made the Stamford Bridge pitch muddy, but City adapted better. England midfielder Colin Bell scored twice in a 3-0 win. Bell was so noted for his stamina that fans nicknamed him "Nijinsky" after the Derby winning racehorse.

"Chelsea were lowered into a watery grave by the men who won the prize the year before," wrote Maurice Smith poetically in the Sunday People. "They were men for whom the 'Bell' tolled." 

Television cameras were present but the action was only shown as highlights on Sunday afternoon.

It was a time when television companies were plagued by industrial action, so the match was screened in monochrome. 

This was perhaps why Chelsea wore their change strip of yellow shirts with blue shorts and City their second choice red and black stripes - to provide a greater contrast for viewers watching in black and white.

Despite the FA Cup defeat, Chelsea still had plenty to play for. They were fourth in the league table but their demanding schedule included leaders Leeds United and eventual champions Arsenal.

To add to their problems, star striker Peter Osgood was about to begin a long suspension.

In early February, the squad boarded a plane for Kingston in Jamaica to face Pele's club Santos in a friendly. Unfortunately, first choice goalkeeper Peter Bonetti aggravated a shoulder injury during the trip.

In March, the European competitions started up again and in the first leg of their quarter-final, Chelsea discovered why Club Brugge had been unbeaten at home for three years.

They were 2-0 down after the first leg but the return at Stamford Bridge proved an electric encounter.

Goals from Peter Houseman and Osgood, at last back after suspension, took the contest into extra time with only nine minutes remaining. Two further goals came and Chelsea were through 4-2 on aggregate.

Meanwhile, City recovered a two goal deficit against old foes Gornik Zabrze, on a "mud bath" pitch at Maine Road.

Instead of a penalty shoot-out,  a playoff match in Copenhagen was arranged which City won 3-1. The draw for the semi-finals had already thrown the two English clubs together.

"We would rather have drawn a foreign team and then met Chelsea in the final, when we might have had a chance of changing the venue to England and giving the final a bit of atmosphere," suggested manager Joe Mercer.

All was not well at Manchester City. Since the turn of the New Year hey had only won one league match and the relationship between Mercer and coach Malcolm Allison was deteriorating.

Then, a Football Association disciplinary panel fined Allison £200 ($283/€232) and banned him from all football for two months as a punishment for comments made to a referee. Allison branded the ban as "terribly severe".

In his newspaper column, England goalkeeper Gordon Banks suggested: "I think it will have quite a lot to do with the result. Malcolm is a wonderful driver of men."

Twelve days before the first leg of the semi-final at Stamford Bridge, City lost three of the four matches they played as fatigue set in.

Chelsea's injury toll was also mounting. Osgood, only just back from suspension, was ruled out.

Into the breach stepped South African born Derek Smethurst. He scored the only goal of the first leg in front of more than 45,000 fans.

Three days later, the sides met in the league at Maine Road. This time only 26,000 watched a 1-1 draw.

That season, some European opponents rearranged domestic matches to prepare for their European ties, in contrast to the schedule faced by both City and Chelsea before the second leg. 

On the preceding Saturday, City lost 2-0 at Stoke. Chelsea beat Coventry 2-1 but influential midfielder John Hollins injured his knee. On Monday, 48 hours before the second leg, City drew 2-2 with Liverpool, but central defender Tommy Booth limped off and left the ground with a bandaged knee to rule him out. 

Meanwhile, Osgood aggravated his ankle injury in Chelsea's home defeat against Burnley.

Reporter Bob Harris, writing in the provincial press, called it "the battle of the crocks". 

"The situation is almost farcical," he moaned.

For one of the biggest matches of the season, City were without six regulars and even drafted in 19-year-old goalkeeper Ron Healey. 

Five big Chelsea names were absent as skipper Ron Harris, nicknamed "Chopper", led them out at Maine Road in front of 43,663 crammed onto the terraces.

The programme, which cost five pence in the newly decimalised currency, contained a team list which was largely a work of fiction. On the cover Manchester City chairman Albert Alexander echoed a familiar theme. "It would have been nice to have met them in Athens for the final," he said.

Three minutes before half-time, Chelsea were awarded an indirect free-kick taken by Keith Weller. Defender David Connor moved in to join the keeper at the near post.

"I tried to bend it around a defender," Weller said. "I bent it too far and Connor seemed to change his mind about heading and put young Healey off. It is ironic that had the ball gone in without Healey touching it, it would have been no goal."

Up in the television gantry, commentator Gerald Sinstadt described how Healy was "caught with one eye on what was happening in front of him".

"Apparently no danger, the goalkeeper catching it, losing it and there, it just trickles inside the post," he said. It proved the only goal of the night and Chelsea were through.

"We couldn't argue about the result," said Mercer. "We didn't play at all in the first half."

Chelsea went on to win the final against Real Madrid but, totally in keeping with the character of the season, this went to a replay.

The only previous final meetings of City and Chelsea were both in English domestic competitions. The most recent came in 2019 when City won on penalties at Wembley after a 0-0 draw in the League Cup.

In Porto tomorrow, UEFA might well prefer an outcome similar to the first final meeting.

In 1986, Chelsea lifted the Full Members Cup after an extraordinary match in which City scored three times in the last five minutes. It finished 5-4 to Chelsea and both sides had played a league match the previous day in the spirit of 1971's crowded schedule

By an ironic twist, the Full Members Cup had only come into existence to fill the void when English clubs and their fans had been banned from playing in Europe because of hooliganism.