Heathrow Ariport, fastest way to St. Pancras

The Tube is the fastest way to get from Heathrow to St Pancras, but it will be a bit tight for your chunnel-train connection. Assuming you you can get past immigration in under an hour, the tube will take 1 hour 15 minutes, getting you to St Pancras at about 12:45. So if everything goes right you'll make it. If your plane doesn't land on time, or there is a longer line than usual at immigration you won't make the chunnel-train check-in.

It looks tight but worth a gamble - unless you are averse to losing the Eurostar tickets. You do have changeable tickets, right? Land 1030, clear immigration and customs with a fair wind in 90 minutes, 15 to find Underground and buy ticket/Oyster, 55-60 on the Tube depending on terminal and train coming right away, its 1315. 15 minutes to find check in for Eurostar, pray that there are no lines to check in - Bingo. I'd be sweating...

You will want the Picadilly line. It runs from Heathrow to the King's Cross station. The King's Cross station is adjacent to St. Pancras. Michael is correct on the travel time for the underground. Even when you arrive at King's Cross, you are probably still almost 8-10min from the Eurostar platform as you will have to transition between the two facilities. If you can find the signage, there is a nice underground pathway between the two stations.

Why did you book a 2:00 Eurostar and can you change it? Why start your trip with such a stressful connection? If the slightest thing goes awry you are up the well known creek.

Since you are already at Heathrow, flying directly from there would be far easier than Eurostar, unless you are heading for somewhere like Calais or Lille.

Probably not enough time. The Eurostar people at St. Pancras insist that you arrive there 30 minutes before the train is scheduled to leave.

You might be ok. There is a faster train- heathrowexpress.com. It claims to get you to the city center in 15 minutes, and then you just take the tube to St. Pancras from wherever it spits you out in London. I haven't researched it fully because I'm poor and this train is more expensive than the tube. But I do know the tube takes FOREVER. (over an hour, as stated before in this post).

Jeanne - I have missed the Eurostar twice - once in London and once in Paris. One time the RER train broke down and another there were at least 100 people in line for the Eurostar and arriving exactly 30 minutes ahead was not getting me on that train. I had non-refundable tickets and was just put on the next train. They didn't seem worried about non-refundable tickets, but I'm not sure if anything has changed as that last incident was a few years ago. If you already bought the ticket, you will probably have to take your chances. If not, I second the flying suggestion. Since you are already at Heathrow give it a good buffer for connection time and take a flight. My experience in Heathrow immigration has either been getting through in 15 minutes or standing line for 1.5 hours... You can just never tell. I'm not sure if the Heathrow Connect train would be a viable option as I think the connection from Paddington to St Pancras takes multiple tube connections and a taxi across town could take just as long. You could try a taxi directly from Heathrow.

As others have said, this connection looks to be...sporty. A further option might be the Heathrow Express. Runs frequently, and gets you into town in ~15 minutes. From Paddington, however, you may be better served getting a taxi vice the Tube. It depends completely on where you want to take the risk - do you take the risk of the Tube being slow, crowded, etc., or do you risk traffic. For reference, Transport for London's Tube journey planner (http://www.tfl.gov.uk/) lists time from Heathrow Terminal 3 to St. Pancras Int'l as either 57 minutes, using the Heathrow Express and Tube from Paddington, or 1 hr 10 minutes using the Piccadily Line.

Sporty isn't the word I'd use but it is kinder. We just did this last month. Heathrow Express then a taxi to St. Pancras. We did figure out that it would have been about the same price (and loads easier) to take a just take a cab. The problem with a cab is the traffic question...how long will it take. The Heathrow Express cuts out that unknown.

Ok...looks like I definitely didnt plan well. My upcoming trip lands at 10:15 at Heathrow and I have booked a nonrefundable ticket on the Eurstar leaving St. Pancras at 1:00. When I traveled the Euro in France there was no check in. We just got on the train and they checked our ticket after it left the station, so I thought this would be the same at St. Pancras. We are taking the Euro to Calais. The only other train was at 4:00 pm and we didn't want to wait that long. My only hope now is to have them book me on the later train at no charge. Does anyone think I can make this connection? Should I try to change the ticket now to the later time that day?

Joanne you could have had your own thread. The OP posted this many months ago and has probably completed her trip but will now get emails about your question. You have a very slim chance of making that train (Eurostar is the train, Euro is the coin). 10:15 touchdown if your plane is on time and goes straight to the gate. 60 minutes for all the airport stuff if you have no luggage; 90 if you do or there are several planes at the same time. You also have to find the ground transport and get tickets and wait for it. Lets be generous and say everything goes like clockwork and the train etc. is pulling in just as you get to the platform (that won't happen). So its 11:45. Piccadilly Line? 12:55 to arrive at St Pancras for a 1300 departure? No Go. You had to be there before 12:30 to check in or they wouldn't let you on. Heathrow Express to Paddington and taxi?
The 11:48 arrives Paddington (except Sundays, if everything is tickty-boo) at 12:03. 10 minutes to get outside a get a taxi. 12:13. Your taxi has to have you outside St Pancras by no later than 12:20 so you can find your way inside and get into the station and find the Eurostar area and through the queues to the check in machine before 12:30. 7 minutes from Paddington to St Pancras? Not likely. Maybe. Unlikely. Almost impossible. Anything faster? Maybe your plane will be early. Maybe you have no luggage and manage to be first in the queue at passport control. Maybe. Anything slower - many many things could slow you down. Better use the toilet on the plane before the seat belt sign comes on because you won't have time after you land.

Thanks for your analysis and comments Nigel. I have already booked a later Eurostar that doesnt leave until 19:30, but I sure paid alot for it! My previous booking was non-transferable of course. Its a lesson learned!